Audit all concurrent types for &mut aliases

[brson]

Followup to #11583

It is incorrect to allow &mut to alias in parallel. Mutable concurrent types must use interior mutability where &mut would result in aliasing.

[steveklabnik]

What do we think the status is of this today? @aturon , I'd imagine that during stability, these kinds of concerns have been addressed, yes?

[alexcrichton]

Yeah I think that all public-facing types have been audited as necessary. A lot of the internal implementation details of channels still need to be dealt with, but that can always happen at a later date!