feat(gg): simplify and make safer resolver type resovler

[jasonkuhrt]

will close #409

Safer

• strings constants are used to specifiy which graphql objects names may be returned.
• null cannot be returned
• undefined cannot be returned

Simpler

• no nesting of type-generics
• only permits returning names of graphql objects (not objects)

References:

• https://github.com/graphql/graphql-js/blob/c1ca546704018e9630dd87db0dbb59d5ac3de501/src/execution/__tests__/abstract-test.js
• https://github.com/graphql/graphql-js/blob/fd308ceb72a90a3903411a830da46adcd1156177/src/type/definition.js#L831-L836