Arity: Handle shadowing properly

In #20070, 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.