Clean up module structure

[jml]

• Move almost all the implementation into GraphQL.Internal (I left the stuff in GraphQL.hs for now, but we could do the same sort of thing)
• Create public re-export modules for the following
  • GraphQL.API
  • GraphQL.Resolver
  • GraphQL.Value
• Remove following semi-internal modules
  • GraphQL.API.Enum
  • GraphQL.Value.ToValue
  • GraphQL.Value.FromValue
• Haddock descriptions for each module (appear on table of contents)

This means our public API surface is just GraphQL, API, Resolver, and Value.

Motivations:

• clarifies our public API surface
• better separation of concerns between "how to implement" and "what is public interface"
• more flexibility to break up resolver, etc. if we want to
• consistency for end users ("if there is an GraphQL.Internal, what does GraphQL.API.Enum mean?")
• less import ceremony for end users

I've manually re-exported all the names, because this makes for better Haddock output.

All the internal modules have not-home set, which means that Haddock won't link to them unless there's no choice.

I would like to do a release before this change is merged.