Trait implementation type elision

[cuviper]

Aha, found my old thread:

Allow type placeholders in trait impls

Could we allow _ type placeholders in fn signatures of trait impls? I understand that type inference is not desired on fn signatures in general, but for a trait impl there is no ambiguity. The types must match the trait declaration exactly, but sometimes this can be annoying to actually write out. For a small example, this is the real BitAnd for HashSet: impl<'a, 'b, T, S> BitAnd<&'b HashSet<T, S>> for &'a HashSet<T, S> where T: Eq + Hash + Clone, S: BuildHasher + Default { …

We should make this happen!