Rollup merge of #22567 - Gankro:unstable, r=alexcrichton

 * Adds features and allows
* Removes unused muts, unused imports, dead code
* Migrates some deprecated code to new io/env
* Changes std::num::uint/int to be re-exports of std::num::usize/isize

libcollections, liballoc, and libcoretest no longer warn during testing.

libstd warns much less, though there's some dangly bits that weren't obvious fixes. In particular, how to only supress deprecated warnings in specific submodules of std.

diff --cc src/libstd/io/mod.rs
Simple merge

diff --cc src/libstd/thread.rs
Simple merge