Grab bag of runtime optimizations

Here are a bunch of of small optimizations that add up to a 36% improvement on one particular message passing [benchmark](https://github.com/brson/rust-sched-bench/blob/master/pingpong.rs).

After this and @toddaaro's optimizations from #8566 the next biggest wins are probably going to be [avoiding the event loop](8586), which is another 25%, and using a [less-allocating channel implementation](8568) (not sure how much this wins but it should be a lot). Beyond that there is still the important optimization of using the stack pointer for TLS instead of the TLS API, reducing lock contention, identifying and reducing other syscalls, page faults, context switches and allocations, recycling tasks. Codegen improvements may help as well as there appears to be some nonsense in the assembly that one wouldn't write by hand.

There are a couple of notable changes here:

* 9692175 Turns `rtassert!` off for optimized builds, adding a new constant that can be queried before running expensive sanity checks: `pub static ENFORCE_SANITY: bool = !cfg!(rtopt) || cfg!(rtdebug) || cfg!(rtassert)`. `cfg(rtopt)` is turned on by makefiles.
* The two commits that reduce lock contention do so by adding unsynchronized checks of the length of queues in strategic places. Under load this makes the SleeperList in particular virtually uncontended (and it was *extremely* contended before).

Before (just with #8566):

```
brian@brian-X1:~/dev/rust-sched-bench$ RUST_THREADS=4 perf stat -- ./pingpong-rust
 Performance counter stats for './pingpong-rust':

       5873.315864 task-clock                #    3.882 CPUs utilized
             9,861 context-switches          #    0.002 M/sec
                23 cpu-migrations            #    0.004 K/sec
             1,438 page-faults               #    0.245 K/sec
    17,321,368,036 cycles                    #    2.949 GHz
    11,409,886,127 stalled-cycles-frontend   #   65.87% frontend cycles idle
   <not supported> stalled-cycles-backend
    13,352,527,802 instructions              #    0.77  insns per cycle
                                             #    0.85  stalled cycles per insn
     3,013,346,012 branches                  #  513.057 M/sec
        47,494,193 branch-misses             #    1.58% of all branches

       1.512948956 seconds time elapsed
```

After (these opts + #8566):

```
brian@brian-X1:~/dev/rust-sched-bench$ RUST_THREADS=4 perf stat -- ./pingpong-rust
Performance counter stats for './pingpong-rust':

       3682.534190 task-clock                #    3.863 CPUs utilized
             5,622 context-switches          #    0.002 M/sec
                24 cpu-migrations            #    0.007 K/sec
             1,432 page-faults               #    0.389 K/sec
    10,979,235,304 cycles                    #    2.981 GHz
     7,260,103,584 stalled-cycles-frontend   #   66.13% frontend cycles idle
   <not supported> stalled-cycles-backend
     8,557,987,829 instructions              #    0.78  insns per cycle
                                             #    0.85  stalled cycles per insn
     1,766,533,692 branches                  #  479.706 M/sec
        38,529,078 branch-misses             #    2.18% of all branches

       0.953177796 seconds time elapsed
```

And here is how Go does on the same benchmark:

```
brian@brian-X1:~/dev/rust-sched-bench$ GOMAXPROCS=4 perf stat ./pingpong-go
 Performance counter stats for './pingpong':

        990.779438 task-clock                #    3.874 CPUs utilized          
               517 context-switches          #    0.522 K/sec                  
                12 cpu-migrations            #    0.012 K/sec                  
               219 page-faults               #    0.221 K/sec                  
     2,949,207,854 cycles                    #    2.977 GHz                    
     1,913,919,758 stalled-cycles-frontend   #   64.90% frontend cycles idle   
   <not supported> stalled-cycles-backend  
     2,672,127,904 instructions              #    0.91  insns per cycle        
                                             #    0.72  stalled cycles per insn
       609,300,846 branches                  #  614.971 M/sec                  
         9,815,707 branch-misses             #    1.61% of all branches        

       0.255722866 seconds time elapsed
```


Here's what the profile looks like after these optimizations, those in #8566, *and* a hacked up optimization to not hit epoll (not included in this PR):

```
+   8.85%  pingpong-rust  pingpong-rust                       [.] rt::comm::__extensions__::try_send_inner_6667::_a19fb8784b62bf94::_0$x2e0
+   7.09%  pingpong-rust  pingpong-rust                       [.] rt::comm::__extensions__::try_recv_6722::_9022caef57e17a4::_0$x2e0
+   5.15%  pingpong-rust  libc-2.17.so                        [.] malloc
+   4.45%  pingpong-rust  libc-2.17.so                        [.] _int_free
+   3.39%  pingpong-rust  pingpong-rust                       [.] std..rt..comm..ChanOne$LT$std..rt..comm..StreamPayload$LT$$LP$$RP$$GT$$GT$::_a4c4e2b19dcf3b21::glue_drop_6336
+   3.30%  pingpong-rust  libpthread-2.17.so                  [.] pthread_getspecific
+   3.08%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_29695::change_task_context::_e41693fc69da04e::_0$x2e8$x2dpre
+   2.62%  pingpong-rust  libc-2.17.so                        [.] _int_malloc
+   2.53%  pingpong-rust  libpthread-2.17.so                  [.] pthread_mutex_lock
+   2.31%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::switch_running_tasks_and_then::anon::expr_fn_29766
+   2.30%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] unstable::atomics::__extensions__::meth_20980::new::_cc85e4d2d8dd7469::_0$x2e8$x2dpre
+   2.12%  pingpong-rust  libpthread-2.17.so                  [.] __pthread_mutex_unlock_usercnt
+   2.04%  pingpong-rust  librustrt.so                        [.] swap_registers
+   1.97%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_29405::run_sched_once::_f3525925b944a51::_0$x2e8$x2dpre
+   1.74%  pingpong-rust  pingpong-rust                       [.] rt::comm::__extensions__::drop_3677::_ae547a17a7ff6ea9::_0$x2e0
+   1.67%  pingpong-rust  libc-2.17.so                        [.] free
+   1.56%  pingpong-rust  pingpong-rust                       [.] rt::comm::oneshot_3615::_5a94a9b33e484f55::_0$x2e0
+   1.54%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_21144::enqueue_task::_ce3b62147171303e::_0$x2e8$x2dpre
+   1.48%  pingpong-rust  libpthread-2.17.so                  [.] pthread_setspecific
+   1.47%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::thread_local_storage::pthread_getspecific::_741ee83c8a1aea64::_0$x2e8$x2dpre
+   1.43%  pingpong-rust  libc-2.17.so                        [.] pthread_mutex_lock
+   1.39%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_29730::schedule_task::_373b465acd34aec7::_0$x2e8$x2dpre
+   1.36%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::local_ptr::unsafe_borrow_32251::_32439a71381e9371::_0$x2e8$x2dpre
+   1.34%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::task::__extensions__::meth_28172::take_unwrap_home::_8ff7f630b5c4cb79::_0$x2e8$x2dpre
+   1.34%  pingpong-rust  pingpong-rust                       [.] _$UP$std..rt..task..Task::_6757b7c895de82::glue_drop_3896
+   1.33%  pingpong-rust  pingpong-rust                       [.] rt::comm::__extensions__::try_recv_6760::anon::expr_fn_6765
+   1.29%  pingpong-rust  pingpong-rust                       [.] ping_pong_bench::run_pair::anon::anon::expr_fn_6583
+   1.25%  pingpong-rust  pingpong-rust                       [.] std..option..Option$LT$$UP$std..rt..sched..Scheduler$GT$::_ef4d43fe1137a657::glue_drop_5755
+   1.22%  pingpong-rust  pingpong-rust                       [.] rt::local::__extensions__::meth_5148::take::_87eba8df7b0468::_0$x2e0
+   1.20%  pingpong-rust  libc-2.17.so                        [.] __memmove_ssse3_back
+   1.19%  pingpong-rust  pingpong-rust                       [.] std..rt..kill..BlockedTask::_89842139b9381497::glue_drop_4308
+   1.16%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::task::__extensions__::meth_28209::is_home_no_tls::_ae80b16046b0aa7d::_0$x2e8$x2dpre
+   1.09%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] libc::funcs::c95::stdlib::free::_f269ec056c867a::_0$x2e8$x2dpre
+   1.09%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_15900::run_task::_ce3b62147171303e::_0$x2e8$x2dpre
+   0.93%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::task::__extensions__::meth_28168::give_home::_13e078f88f4164a::_0$x2e8$x2dpre
+   0.86%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_15988::deschedule_running_task_and_then::_6dd6f0e0a6b272fe::_0$x2e8$x2dpre
+   0.86%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] unstable::sync::__extensions__::get_29463::_35e2217cc3c2555b::_0$x2e8$x2dpre
+   0.83%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::thread_local_storage::set::_1d5c2ea6f9affc47::_0$x2e8$x2dpre
+   0.80%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::resume_task_immediately::anon::expr_fn_29762
+   0.80%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::meth_29407::interpret_message_queue::_e6191113fdd2ee81::_0$x2e8$x2dpre
+   0.74%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::sched::__extensions__::run_task::anon::expr_fn_29741
+   0.74%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::work_queue::__extensions__::pop_29570::anon::expr_fn_29596
+   0.72%  pingpong-rust  libstd-6c65cf4b443341b1-0.8-pre.so  [.] rt::kill::__extensions__::meth_15714::wake::_3ccd357d3a209434::_0$x2e8$x2dpre
```