Guaranteed correct name in AST

[jml]

• Move all the name stuff from Value to AST
• Guarantee that only valid names ever appear in the AST or anywhere else
• Add a HasName type class that is implemented by all named things in the schema (can be extended to other things later, this is just the only place I needed it)
• Change the getFooDefinition methods to return Either NameError Foo. This involves changing most of the code to be applicative, which in turn involves a fair chunk of using the composition operator. Sorry.
• While doing that, removed some duplication (mostly because it was less boring than typing the same thing twice)
• Update Server module to use the API module and to get names from there, rather than directly from symbols. This simplifies some of the types (but I tried to tread lightly here). Dodgy names are now thrown as QueryError. I think we can do better than this, but await your error handling branch.

New TODOs:

src/GraphQL/API.hs:
  -- TODO(jml): Given that AnnotatedType is parametrised, we can probably reduce
  -- a great deal of duplication by making HasAnnotatedType a parametrised type
  -- class.

src/GraphQL/API.hs:
  -- TODO(jml): Be smarter and figure out how to say "all integral types" rather
  -- than listing each individually.

src/GraphQL/Server.hs:
  -- TODO(jml): I don't understand why I can't extract (Object typeName interfaces fields)
  -- from this Union instance as I did for the above Union instance.

[jml]

Should point out that you can tell where we can use more structure in error type by searching for formatNameError.

[jml]

Build failure is CircleCI being buggy.

In the end I've made very few changes because I'm not clear what the actionable feedback is.

[teh]

Survived Christmas :)

Can I merge this?

[jml]

Merged. Sorry for the delay.