Move Name to Value module and make invalid Objects impossible

[jml]

I started out moving Name to the Value module with the intent of not adding any new TODOs. I ended up adding three, with a much bigger PR than I expected.

That's because I tried to eliminate the TODO that asked What the heck does unionObject do? I wrote a test to figure it out and found that it constructed invalid objects--objects with duplicate field names. I really don't want to ever have that bug again, so I jotted down a property and updated the types to give Object smart constructors so it's now only possible to construct Objects with unique field names.

To be honest, I still don't understand why unionObjects is the right behaviour in the union resolver, but I'm certain that its old behaviour was definitely wrong.

• Start of tests for GraphQL.Value
• Remove duplication of Name from Schema and Value, moving everything to Value
• Create ObjectField type for object fields
• Rename Map to Object, as per the TODO comment and spec
• Change some data to newtype (hlint warning, removes runtime overhead)
• Remove unnecessary parens (hlint)
• Extract wrongType helper (we can have top-level private functions now that we have explicit export lists)

[jml]

PTAL

[jml]

Sure, I meant that if we validate the query there would be no need to check for duplicate fields. Even if the Object defines duplicate fields (which is another interesting bug we should catch, see discussion below) then we'd never hit this condition.

Ah, yes. I make a comment about this in #26.