Rollup merge of rust-lang#146826 - bluurryy:impl-allocator-for-mut-a, r=scottmcm

Implement `Allocator` for `&mut A` where `A: Allocator + ?Sized`

This is a breaking change to the unstable `allocator_api` feature. Tracking issue rust-lang#32838.
It implements the accepted api change proposal from rust-lang/libs-team#508.

The code for the blanket implementation is the same as the one for `&A`, just with the `mut` added.

Author: matthiaskrgr