Add TryFromBytes trait

`TryFromBytes` can be implemented for types which are not `FromZeroes`
or `FromBytes`; it supports performing a runtime check to determine
whether a given byte sequence contains a valid instance of `Self`.

This is the first step of #5. Future commits will add support for a
custom derive and for implementing `TryFromBytes` on unsized types.

TODO:
- More thorough tests for non-FromBytes types (bool, char, etc)
- Document TryFromBytes requirements regarding UnsafeCell
- Remove now-unnecessary safety comment on
  `impl_try_from_bytes_testable!` (both definition and usage sites)
- Remove the TODO comment on the definition of `unsafe_impl!`
- Remove the TODO to test the failure branch of `unsafe_impl!` using
  trybuild

Makes progress on #5