Python's regex engine performs pathologically on regexes like
'<!--.*?-->' when fed a large string of repeating '<!--' clauses.
Thanks to JaimeSlome / security@huntr.dev for the report; closes#1380.
Solved by switching to the Rust implementation, which does not suffer
from this issue.
entsToText(), minimizeHTML(), and the old regex constants have been
removed; they do not appear to be used by any add-ons.