core lib request: Add a count/length method to `core::range::Range`

The old core::ops::Range was an iterator directly, so you could check the count with r.count(). On the new range type you can't easily check the count / length of the range (eg: to compare it to the length of a string when you're unit testing your parser). writing r.into_iter().count() seems a little overkill to me.

I do understand that the point of the new Range is to not natively be an iterator so that it can be Copy, but it still logically has a length to it, and checking that length is a non-destructive step, so having this method built directly onto the type shouldn't break anyone's assumptions on accident, and it seems completely in the spirit of the existing is_empty method.

Start at the core::range::Range entry point and compare the requested query with the existing is_empty method. Resolve whether the API should be named count or length and define its behavior for representative ranges; done means the non-destructive query is available with coverage for those cases.