Specifically to avoid any more hidden ReDoS in those regexes.

Seems to run tests in 6.9s vs 7.0s so might be a bit of a speed bonus on
some platforms!

Replaces a bunch of `[^\d]*$` with bounded `(?:[^\d]|$)`

Double checked the RFC6265 spec: time cannot have non-digits beside the
colons.

None of the regexps (at least, when they were removed) are vulnerable to
ReDoS. However, took this opportunity to check that the RFC is being
closer and more clearly documented where in the code.

Another way to put this: "regexps are magic and hinder code analysis"

Introduced some equivalence tests to ensure that certain "weird" dates
are indeed parsing the same as their "canonical" RFC6265 counterpart.