A new scan of irregularities and errors in regexps in the Emacs tree, using an experimental version of relint/xr.
New checks include ones specific to file names, more elaborate subsumption checks, and rx-specific checks. All are designed to avoid false positives; comments are very welcome!

For an explanation of individual diagnostics, see
https://github.com/mattiase/relint/blob/master/README.
Do ask if you find anything unclear.

(I will take care of those in rx-tests.el; they probably just need suppressing.)