core::vec is missing methods for mutable slices

[brson]

There just aren't any implementations on &mut [T] at all. mut_slice is an obvious one. also swap. I could also use a method for copying elements from one &[T] to an &mut [T].

[ntrel]

mut_slice seems to be here:
https://github.com/mozilla/rust/blob/master/src/libcore/vec.rs#L281

[brson]

@ntrel that's a function though, so must be used like vec::mut_slice(foo, ...), when I would prefer to use it like foo.mut_slice(...)

[brson]

Nominating backward compat

[graydon]

accepted for backwards-compatible milestone