Use macro-generated impls even for foreign types

[Kevin Reid]

Make a macro that can be used to generate an implementation for another crate's type just like the derive macro does for a local one, and use it to implement Exhaust for types like Option, Result and Ordering.

This will ensure that our impls can have all the expected functionality and be tidily opaque rather than an aggregation of iterator combinators.

Open question: Should we rigorously use such implementations, or keep using iter::once, iter::empty, and such for simple cases?

[Kevin Reid]

Another possibility would be to wait for Rust to stabilize RFC 2515 type_alias_impl_trait, at which point we can just use

impl Exhaust for SomeType {
    type Target = impl Iterator<Item = SomeType>;
    ...
}

for every impl Exhaust that isn't intending to expose its iterator type.