Add a copy-on-write container.
Added Freeze trait to CowArc
Allow SIMD types in generics. Closes #10604
Rename reserve to reserve_exact and reserve_at_least to reserve

Changes in std::{str,vec,hashmap} and extra::{priority_queue,ringbuf}.
Fixes #11949
extra: Move uuid to libuuid
cc #8784
auto merge of #11912 : poiru/rust/8784-libuuid, r=alexcrichton

cc #8784
Simd feature gating + Win32/64 fixes.
Register new snapshots
Added missing xfail-fast.
auto merge of #11717 : DiamondLovesYou/rust/master, r=alexcrichton

Note that it still doesn't allow generic types to be marked with #[simd].
auto merge of #11951 : dmanescu/rust/reserve-rename, r=huonw

Changes in std::{str,vec,hashmap} and extra::{priority_queue,ringbuf}.
Fixes #11949
auto merge of #12026 : alexcrichton/rust/snapshots, r=cmr
auto merge of #11230 : csherratt/rust/cow, r=alexcrichton

This allows patch adds a new arc type that allows for creation of copy-on-write data structures. The idea is that it is safe to mutate any data structure as long as it has only one reference to it. If there are multiple, it requires cloning of the data structure before mutation is possible.