Add lru_cache to libextra
Disable priv in std::comm::Port, etc

It's useful to allow users to get at the internal std::rc::comm::Port,
and other such fields, since they implement important traits like
Select.
Fix documentation typo (divison operator is not backslash)
auto merge of #10825 : octurion/rust/master, r=alexcrichton
auto merge of #10211 : ktt3ja/rust/add-lrucache, r=brson

There's an open issue ([Issue #4988](https://github.com/mozilla/rust/issues/4988?source=cc)) for adding an LRU Cache to the standard library. I'm new to this so I hope I didn't miss anything I'm supposed to do.
auto merge of #10817 : alexcrichton/rust/sched-fix, r=brson

Right now, as pointed out in #8132, it is very easy to introduce a subtle race
in the runtime. I believe that this is the cause of the current flakiness on the
bots.

I have taken the last idea mentioned in that issue which is to use a lock around
descheduling and context switching in order to solve this race.

Closes #8132
auto merge of #10562 : ongardie/rust/master, r=brson

It's useful to allow users to get at the internal std::rc::comm::Port,
and other such fields, since they implement important traits like
Select.

See [rust-dev] "select on std::comm::Port and different types" at https://mail.mozilla.org/pipermail/rust-dev/2013-November/006735.html for background.
Rewrite std::comm

* TODO: performance numbers
* Streams are now ~3x faster than before (fewer allocations and more optimized)
    * Based on a single-producer single-consumer lock-free queue that doesn't
      always have to allocate on every send.
    * Blocking via mutexes/cond vars outside the runtime
* Streams work in/out of the runtime seamlessly
* Select now works in/out of the runtime seamlessly
* Streams will now fail!() on send() if the other end has hung up
    * try_send() will not fail
* PortOne/ChanOne removed
* SharedPort removed
* MegaPipe removed
* Generic select removed (only one kind of port now)
* API redesign
    * try_recv == never block
    * recv_opt == block, don't fail
    * iter() == Iterator<T> for Port<T>
    * removed peek
    * Type::new
* Removed rt::comm