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
windows: Fix several tests on 64-bit.

Signed-off-by: Peter Atashian <retep998@gmail.com>
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.)
Use system rustc if configured with --enable-local-rust

This commit makes the configuration system autodetect a rustc that
is already installed and use that instead of downloading a snapshot.
fix grammar in Vec.retain's doc comment
Fix typo
Guide: tasks
Guide: strings
windows: Fix INVALID_HANDLE_VALUE
Made INVALID_HANDLE_VALUE actually a HANDLE.
Removed all useless casts during INVALID_HANDLE_VALUE comparisons.

Signed-off-by: Peter Atashian <retep998@gmail.com>
Cleanup collections::slice documentation.

This does a few things:

 - remove references to ~[] and the OwnedVector trait, which are both
   obsolete
 - correct the docs to say that this is the slice module, not the vec
   module
 - add a sentence pointing out that vectors are distinct from Vec
 - remove documentation on Vec.

closes #15459
auto merge of #16279 : nham/rust/fix_slice_docs, r=alexcrichton

This does a few things:

 - remove references to ~[] and the OwnedVector trait, which are both
   obsolete
 - correct the docs to say that this is the slice module, not the vec
   module
 - add a sentence pointing out that vectors are distinct from Vec
 - remove documentation on Vec.

closes #15459
auto merge of #16309 : steveklabnik/rust/guide_tasks, r=brson

I wasn't 100% sure of what level of detail I wanted to go into things here. For example, 1:1 vs M:N tasks seems like a better distinction to leave to the Guide.
auto merge of #16321 : gioele/rust/use-local-rust, r=brson

This commit makes the configuration system autodetect a `rustc` that
is already installed and use that instead of downloading a snapshot.
Move system header includes above valgrind.h include

This allows rustc to be build under msys2's mingw64 gcc
auto merge of #16325 : froydnj/rust/vec-grammar-fix, r=alexcrichton

Just a small typo noticed while reading through documentation.
Register new snapshot 12e0f72
auto merge of #16349 : nikomatsakis/rust/snapshot-20140808, r=nikomatsakis
auto merge of #16327 : mdinger/rust/typo, r=steveklabnik

Fix typo. It's possible it's `These modules` but I think it's supposed to be singular because it's not refering to nested modules.
Guide: method syntax