Add transmute_ref! and transmute_mut! macros

[joshlf]

We currently have a transmute! macro which safely transmutes an AsBytes type into a FromBytes type. We should add macros which do the same for mutable and immutable references.

A candidate implementation of transmute_ref! is in #183. We may also want to ditch it in favor of something based on this approach suggested by @gootorov, but we'd need to first confirm that invalid uses are guaranteed to generate compile-time rather than runtime errors (rust-lang/rust#112090). EDIT: Based on the discussion there, it seems that there is such a guarantee, but we'd need to be careful about how we structure things to make sure that we actually write code that benefits from that guarantee.

Note that rust-lang/rust#112301 will likely make @gootorov's technique a bit annoying for users (although the error messages are way better than the existing technique, and the implementation is way simpler, so IMO it's still worth it on balance).

EDIT: Another issue with this approach is that generic APIs might never fail until they're used by a downstream crate since monomorphization only happens at that point. One potential solution would be to keep the functions #[doc(hidden)] and provide macros to invoke them which also call mem::transmute in order to ensure that they can only be called from type-concrete contexts. This would artificially hamstring the API, but the footgun is big enough that it might be worth it.

[joshlf]

Closed in #183 and #550.