When inductives are indirectly mutually recursive, say `inductive T | t
(args: List T)` instead of `inductive T | t (arg : T)`, Lean would fail
to find an instance of Hashable for `T` because it was not yet defined.
This commit makes sure that the deriving handler adds a needed Hashable T
instance to the local scope so that Hashable.hash can be resolved
recursively.
