Protocol unification & Add Rest XML deserialization support (#369)

* Refactor HttpBinding protocol

* Update XmlBindingTraitParserGenerator to fix new protocol tests

* Clear all warnings

* Major overhaul of Http Trait based protocols

* Remove pointless wrapping

* Nullable types fall through

* Cleanup JsonParserGenerator, refactor

* More Json parser generator cleanups

* Update test

* Fix unit test

* [cleanup] Move interfaces to their own files

* Use parser / serializer generators from AwsJson