AST: Represent OpaqueTypeDecls.
To represent the abstracted interface of an opaque type, we need a generic signature that refines
the outer context generic signature with an additional generic parameter representing the underlying
type and its exposed constraints. Opaque types also need to be keyed by their originating decl, so
that we can treat values of the same opaque type as the same. When we check a FuncDecl with an
opaque type specified as its return type, create an OpaqueTypeDecl and associate it with the
originating decl. (A representation for *types* derived from the opaque decl will come next.)
