Implement `VecReadWrapper` for `MaybeReadable`

Implement `VecReadWrapper` for `MaybeReadable`

This makes it much simpler to deal with `MaybeReadable` types in
`Vec`s in TLVs as we can transparently deal with them as `vec`,
with the wrapper doing the Right Thing.

This requires we implement `MaybeReadable` for all `Readable` which
has some downstream implications, but nothing too bad.