Replace substitution function with a map

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.

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to
   can also be definitions.
 - Move all AST data to `AST.Document` and reexport them.
 - Rename `AST.OperationSelectionSet` to `AST.Document.SelectionSet`.
+- Make `Schema.Subs` a `Data.HashMap.Strict` (was a function
+  `key -> Maybe value` before).
 
 ### Removed
 - `AST.Field`, `AST.InlineFragment` and `AST.FragmentSpread`.