Make sure type inference with `a..b` as good as `range(a,b)`

Make sure type inference with `a..b` as good as `range(a,b)`

The new `::ops::Range` has separated implementations for each of the
numeric types, while the old `::iter::Range` has one for type `Int`.
However, we do not take output bindings into account when selecting
traits. So it confuses `typeck` and makes the new range does not work as
good as the old one when it comes to type inference.

This patch implements `Iterator` for the new range for one type `Int`.
This limitation could be lifted, however, if we ever reconsider the
output types' role in type inference.

Closes #21595
Closes #21649
Closes #21672