Add `Initialize`/`Inhabited` (`TryFromBytes` super-trait)

[joshlf]

A number of desired features require knowledge of the recursive layout of a type down to individual fields. KnownLayout is not recursive (e.g. you can derive KnownLayout on a sized type whose fields are not KnownLayout), and so we cannot use KnownLayout without a semver breaking change. Besides, KnownLayout is philosophically the wrong place for these features.

These features include:

• In-place initialization (Support incremental in-place initialization #2749)
• Field projection (Support field projection in any #[repr(transparent)] wrapper type #196)
• Zeroing padding (Provide the ability to zero padding bytes and return &Initialized<T> #494)
• Access to a value's initialized prefix (Retrieving prefix bytes of types with trailing padding #2668)

Instead, we should add a super-trait to TryFromBytes which has a recursive requirement (just like TryFromBytes) but which is not as heavy-weight as TryFromBytes (which requires deriving an entire validator) and does not expose the ability to construct a type to users outside of a module (which is something that an author may wish to avoid). Naming TBD (Initialize and Inhabited were both proposed, but other names may be better).

Open questions

• Could we migrate any functionality from KnownLayout, thus unlocking this functionality a) as a dependency of this new trait or, b) for types which KnownLayout can't support?