Traits other than NoCell permit UnsafeCells

TODO:
- Require `T: NoCell` and `U: NoCell` in `transmute_ref!` and
  `transmute_mut!`
- Tests
  - Exercise transmuting `UnsafeCell`s by value
- Fix compilation with some `AsBytes` methods
- Time to rename `AsBytes` to `ToBytes`?

Closes ##251