Support a copying read from Unalign<T> where T: !Copy but is FromBytes + IntoBytes

[kupiakos]

When working with generic code where we don't want to require Copy for large structs but is still constructed from unaligned bytes, the Ref::read method comes in handy. However, this doesn't work when you have a Ref<&[u8], [Unalign<T>]>, since there's no builtin way to index a Ref<_, [T]>.

It'd be very nice if I could take some &Unalign<T> and copy out a T where T: FromBytes + IntoBytes. This would be infallible shorthand for T::read_from_bytes(unaligned_val.as_bytes()).unwrap(). I'm not aware of an infallible way to copy out of an Unalign<T> where T: !Copy.

[joshlf]

I'm going to make this a child of #1316 - the full solution to this is to support generic types in transmute!, which in turn requires being able to express generic size equality.