Make sure type-special-casing for numbers is updated to account for extension types.

Extension types allow introducing subtypes of `num` and `int`.
We should make sure that the special-case rules for typing of `int + int` are aware of this, and figure out what we want them to do when meeting a surprising subtype of `int`.

If the rules usees phrasing as "is a non-`Never` subtype of `int`", that's no longer guaranteed to be the type `int` or a type variable with bound or promotion to `int`, or necessarily use the `operator+` of `int`.

We may just have to add something like "and the `+` operator an instance member, not an extension type member" to discount a subtype which shadows the instance method with an extension method.

But first we should look at the rules, figure out if there is a problem, and whether or not, if they should be changed.
(We may *want* extension type subtypes of `int` to work in some cases, where they wouldn't with the current phrasing. Impossible to know without checking.)