Auto merge of #23952 - Kimundi:more_string_pattern, r=alexcrichton

This adds the missing methods and turns `str::pattern` in a user facing module, as per RFC.

This also contains some big internal refactorings:
- string iterator pairs are implemented with a central macro to reduce redundancy 
- Moved all tests from `coretest::str` into `collectionstest::str` and left a note to prevent the two sets of tests drifting apart further.

See #22477

Author: bors