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.
