Allocators are stable on nightly Rust

[TDecking]

rust-lang/rust#156882 has been merged. The first stable Rust version with stabilized allocators is 1.100, which is due in November.

I don't think that this should influence API and feature design right now, but Rust 1.100 should obviously be the target for any future MSVR bump.

This issue is mostly a reminder.

[alejandro-vaz]

note: update ./README.md stating that the allocator-api feature is not nightly but 1.100+

MSRV 1.100 will definitely not arrive soon

[alejandro-vaz]

thanks @TDecking for the update