Wrapper newtypes will generate custom implementations that will defer to
the inner type's JSON traits instead of deriving to get them.
This gives us much nicer JSON implementations allowing directly `T`
instead of `{ inner: T }`.

Bytes newtypes will have a specialization that serializes to hex
bytestring instead of the `[number]` array that would otherwise be used.

Introduces `@custom_json` comment DSL for newtypes that tell
cddl-codegen to avoid generating/deriving any JSON traits under the
assumption that some custom trait impls will be provided post-generation
by the user.