Group encoding packages under common namespace to improve shared logic

[jasdel]

It would be helpful if the encoding packages like json, xml, query, etc, where grouped under a common encoding pkg. This makes it easier for these packages to have a common internal pkg containing shared behavior such as encodeFloat.

Propose a pkg refactor similar to:

/smithy-go
     /encoding
          /json
          /xml
          /query
          /httpbinding (aka rest)
          /eventstream
          /...
          /internal
               /float.go
               /...
      /transport
          /http
          /mqtt
          /eventstream
          /...

[jasdel]

Fixed in, #242