Case-insensitive environment keys.
Remove unstable marker from std::rt

The `std::rt` module was marked `unstable` [a while
back](https://github.com/rust-lang/rust/commit/b6d4d117f4c2770649c7ddc2ad9ad4ce4c3b13b1),
and this change was not reverted when we moved to an `experimental`
baseline for `std`.
stop clamping the DWARF version to 3 on Linux

Closes #13611
Pacify lint gods.
auto merge of #17349 : aturon/rust/rt-experimental, r=alexcrichton

The `std::rt` module was marked `unstable` [a while back](https://github.com/rust-lang/rust/commit/b6d4d117f4c2770649c7ddc2ad9ad4ce4c3b13b1), and this change was not reverted when we moved to an `experimental` baseline for `std`.
auto merge of #17364 : thestinger/rust/debuginfo, r=pnkfelix

Closes #13611
auto merge of #17249 : vadimcn/rust/env-keys, r=alexcrichton

Closes #16937
configure: Remove support for i686-pc-mingw32 triple
mk: Remove check-fast target
jemalloc: Un-revert accidentally reverted jemalloc
auto merge of #17330 : brson/rust/bot-house-cleaning, r=alexcrichton

Both these things were transitionary until bot upgrades.
Allow syntax extensions to return multiple items, closes #16723.

This patch replaces `MacItem` with `MacItems`.