codec: improve performance 5-20%, and support finalized msgpack spec.

Performance improvement came from using integer keys in maps where reflect.Type
keys were previously used. Gave 5-20% improvement as this was called at every
iteration of recursive function (encode/decode).

Also, support newly finalized msgpack spec, with support for extensions,
Str8, etc. Full binary compatibility flags exist as before (RawToString, WriteExt).

Also minor clean up (comments, docs, etc).