Arity: Handle shadowing properly

In #20070 (closed), we noticed that findRhsArity copes badly with shadowing. A simple function like g_123 x_123 = x_123, where the labmda binder shadows, already regressed badly. Indeed, the whole arityType function wasn't thinking about shadowing at all.

I rectified that and established the invariant that ae_join and am_sigs should always be disjoint. That entails deleting bindings from ae_join whenever we add something to am_sigs and vice versa, which would otherwise be a bug in the making.

That should fix (but I don't want to close it) #20070 (closed).