Move marshalling code in own module namespace

[mmhat]

The Dhall module is somewhat huge and I hereby propose to split it. Since most of the code deals with marshalling I think that would be a good point to start:

• Dhall.Marshal.Decode
  Decoding stuff goes here
• Dhall.Marshal.Decode.Generic
  Generic decoding stuff goes here (GenericFromDhall + instances)
• Dhall.Marshal.Decode.Instances
  Builtin instances for decoding go here
• Dhall.Marshal.Encode
  Encoding stuff goes here
• Dhall.Marshal.Encode.Generic
  Generic encoding stuff goes here (GenericToDhall + instances)
• Dhall.Marshal.Encode.Instances
  Builtin instances for encoding go here

I think this would improve the hackabilty of the Dhall module a lot. The Dhall module should reexport those modules.

[Gabriella439]

@mmhat: Yeah, that's fine with me as long as the Dhall module still re-exports the functionality