Returning resolvers from other resolvers isn't supported anymore. Since
we have a type system now, we define the resolvers in the object type
fields and pass an object with the previous result to them.
It makes using variables with queries more approachable, but some work
still has to be done.
- The type `Subs` should be renamed and moved out of `Schema`, together with
`AST.Core.Value` probably.
- Some kind of conversion should be possible from a user-defined input
type T to the Value. So the final HashMap should have a type like
`HashMap name a`, where a is an instance of a potential typeclass
InputType.
Language.GraphQL.AST.Transform is an internal module. Even though it
works with the AST, it is a part of the execution process, it translates
the original parser tree into a simpler one, so the executor has less
work to do. Language.GraphQL.AST should contain only the parser and be
independent from other packages, so it can be used on its own.
