Auto merge of #112229 - clarfonthey:range-iter-count, r=dtolnay

Specialize count for range iterators

Since `size_hint` is already specialized, it feels apt to specialize `count` as well. Without any specialized version of `ExactSizeIterator::len` or `Step::steps_between`, this feels like a more reliable way of accessing this without having to rely on knowing that `size_hint` is correct.

In my case, this is particularly useful to access the `steps_between` implementation for `char` from the standard library without having to compute it manually.

I didn't think it was worth modifying the `Step` trait to add a version of `steps_between` that used native overflow checks since this is just doing one subtraction in most cases anyway, and so I decided to make the inclusive version use `checked_add` so it didn't have this lopsided overflow-checks-but-only-sometimes logic.