Direct support for embedded CBOR-in-CBOR

[dcoutts]

This is the tag 24 stuff.

This should just be an optimisation. The semantic change is already
available as encodePreEncoded :: B.ByteString -> Encoding

This branch was done some time ago. It looks fine as far as it goes.

TODOs it will need:

• to be rebased,
• have the API version numbers in the haddock comments updated, and
• most importantly tests and benchmarks adding. The tests should show that it's equivalent to the normal alternative. The benchmarks should show that it's faster than the normal alternative.