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.
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.
It is not a schema (at least not a complete one), but a resolver list,
and the resolvers should be provided by the user separately, because the
schema can originate from a GraphQL document. Schema name should be free
to provide a data type for the real schema later.
This replaces the most usages of MonadPlus, which is not appropriate for
the resolvers, since a resolver is unambiguously chosen by the name (no
need for 'mplus'), and the resolvers are often doing IO.
