Implement higher order template application rule matching

Addresses #22465 (closed)

The current status is that it is almost completely finished. This just needs a review and one final point to work out.

Some things still need to be done:

• Write a Note
• Write regression tests
• Update User Guide section on rewrite rules
• Update release notes
• Check the tv & id substitution code I've written, specifically: rn_subst, tv_subst, and subst_v (help wanted)
• Figure out what to do with _mco (help wanted)
• Check if floating lets or coercions need special handling (help wanted)
• Decide what to do with polymorphic template variables (help wanted) and finish the wrinkle