Custom global allocators (RFC 1398, RFC 1974)

[matthewjasper]

Custom allocators are mostly a library feature, but #[global_allocator] is not. See rust-lang/rust#27389 rust-lang/rust#32838, rust-lang/rust#42774.

• Global allocator attribute - stub
• Maybe add Alloc as a special trait

[alercah]

I think you have that backwards? #[global_allocator] is the language feature, right?

[Havvy]

This feature is stable now. There's a stub of documentation in attributes.md.