make rustdoc more responsive

* move some sidebar contents to a title bar when small
* inline description toggle when small
* make out-of-band and in-band content share space, rather than float and clash
* compress wording of out-of-band content to avoid line-wrap as much as possible
libcollections: Fix RingBuf growth for non-power-of-two capacities
rustc: gensym the module names for --test to avoid introducing user-accessible names.

This requires avoiding `quote_...!` for constructing the parts of the
__test module, since that stringifies and reinterns the idents, losing
the special gensym'd nature of them. (#15962.)
enable PIE by default on Linux for full ASLR

Rust already builds all code as position independent by default, so the
linker can be told to build a position independent executable if it's
not disabled with `-C relocation-model=dynamic-no-pic`. Position
independent code does have a significant cost on i686 (not on x86_64 or
ARM) but there's no significant cost to linking code that's already
position independent as a position independent executable.

Address space layout randomization makes exploiting vulnerabilities much
more difficult by providing a statistical defence against an attempt to
find or modify existing code / data. Without ASLR, it's trivial to use a
vulnerability to take over control of the process via return-oriented
programming.

Rust code can be used for return-oriented programming whether it is safe
or unsafe, so even a fully safe application needs to be built as a
position independent executable to defend against vulnerabilities in
unsafe blocks or C libraries.

Sample program:

    extern crate libc;

    use std::mem;

    static mut global: u32 = 5;
    static constant: u32 = 5;
    fn foo() {}

    fn main() {
        let local = 5;
        println!("stack: {}, global: {}, constant: {}, fn: {}, lib fn: {}",
                 &local as *const u32,
                 unsafe { &global as *const u32 },
                 &constant as *const u32,
                 unsafe { mem::transmute::<_, *const ()>(foo) },
                 unsafe { mem::transmute::<_, *const ()>(libc::mprotect) });
    }

Before:

    stack: 0x3ff15eb9f94, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x32749547530
    stack: 0x3b5d47d80e4, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x394469a7530
    stack: 0x3fe2c4e5564, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x399734a2530
    stack: 0x3e525e0fb24, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x2f62a810530
    stack: 0x3b50fb3eae4, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x2e590e86530

After:

    stack: 0x38cf12c90a4, global: 0x3e2d46b488, constant: 0x3e2d23cf80, fn: 0x3e2d1c2510, lib fn: 0x2617d3b4530
    stack: 0x3d733faf474, global: 0x7eb1839488, constant: 0x7eb160af80, fn: 0x7eb1590510, lib fn: 0x32d30c1f530
    stack: 0x3bb42212ec4, global: 0x5bbb365488, constant: 0x5bbb136f80, fn: 0x5bbb0bc510, lib fn: 0x3595e6c1530
    stack: 0x39f678c1ab4, global: 0x22c4e3c488, constant: 0x22c4c0df80, fn: 0x22c4b93510, lib fn: 0x3835b727530
    stack: 0x3afb25bd394, global: 0x493eab2488, constant: 0x493e883f80, fn: 0x493e809510, lib fn: 0x3478d6a7530

This may also be necessary on other platforms, but I can only test on
Linux right now. Note that GDB gained support for debugging position
independent executables in version 7.1 (March 2010).
rustdoc: Set a nonzero exit status on failure

Now that rustdoc is spawning a child task, the program won't exit with a default
error code if the main task fails (because it never fails). This commit forces
the main task to wait for a child task in order to correctly propagate failure.

Closes #16341
Implement Win64 system ABI.
Move system header includes above valgrind.h include

This allows rustc to be build under msys2's mingw64 gcc
Adding test for issue #15917 which was previously fixed with #15709
libcore: Fix documentation comment for f32.
don't translate items when monomorphizing foreign-ABI functions
auto merge of #16277 : Gankro/rust/responsive-docs, r=cmr

* move some sidebar contents to a title bar when small
* inline description toggle when small
* make out-of-band and in-band content share space, rather than float and clash
* compress wording of out-of-band content to avoid line-wrap as much as possible

## [Live Version Here](http://cg.scs.carleton.ca/~abeinges/doc/index.html)

Pages Of Interest:
* [Vec](http://cg.scs.carleton.ca/~abeinges/doc/std/vec/struct.Vec.html) (small path)
* [TreeSet](http://cg.scs.carleton.ca/~abeinges/doc/collections/treemap/struct.TreeSet.html) (long path)
* [std](http://cg.scs.carleton.ca/~abeinges/doc/std/index.html) (for stability dash)

TBD in a future PR is to convert links in the sidebar into a series of nest ul/li's, so that they can easily be moved to a drop-down in the new title bar. I think this is out of scope for this PR, but am willing to implement it now if desired.
Make small doc contributions less onerous/intimidating
mk: Don't emit landing pads in stage 0.

Reduces time to build stage0 from 11:30 to 9:40 on my machine.
auto merge of #16314 : Ryman/rust/ringbuf_non_pow2, r=huonw

See test for details.
move a test into a run make, to check external affect rather than
implementation details.

(Mainly to avoid accessing the secret internal test module symbol name.)
testsuite: implement #[reexport_test_harness_name] to get access to the
default entrypoint of the --test binary.

This allows one to, e.g., run tests under libgreen by starting it
manually, passing in the test entrypoint.
auto merge of #15964 : huonw/rust/gensym-test, r=alexcrichton

This requires avoiding `quote_...!` for constructing the parts of the
__test module, since that stringifies and reinterns the idents, losing
the special gensym'd nature of them. (#15962.)
auto merge of #16323 : c-nixon/rust/master, r=alexcrichton

This allows rustc to be built with msys2's mingw64

Fixes #16347.
librustc: Allow mutation of moved upvars.
librustc: Update unused mut lint to properly track moved upvars.