configure: Add MINGW64 OS type

MSYS2 supports `MINGW64` system for 64-bit environment. It sets
`MSYSTEM=MINGW64` environment variable, which changes output of
`uname -s` thus affects `configure` behavior.

This patch adds `MINGW64*` support for `configure`.
auto merge of #13199 : klutzy/rust/msys2, r=cmr

MSYS2 supports `MINGW64` system for 64-bit environment. It sets
`MSYSTEM=MINGW64` environment variable, which changes output of
`uname -s` thus affects `configure` behavior.

This patch adds `MINGW64*` support for `configure`.
Use new inner attribute syntax in test framework
auto merge of #13200 : sfackler/rust/test-attr, r=cmr
Removed deprecated functions `map` and `flat_map` for vectors and slices.
auto merge of #13203 : Kimundi/rust/de-map-vec3, r=cmr

They required unnecessary temporaries, are replaced with iterators, and would conflict with a possible future `Iterable` trait.
Updated references to extra in libcollections docs
auto merge of #13208 : Tohie/rust/master, r=alexcrichton
Support new attribute syntax in vim syntax file
auto merge of #13215 : sfackler/rust/vim-attr, r=alexcrichton