Parse reference datetime and timedelta from the same string

[jfinkels]

Change the parse_datetime() function so that it parses both a
reference date and a time delta from one string. The new implementation
attempts to parse the datetime from the longest possible prefix of the
string. The remainder of the string is parsed as the time delta. This
allows us to parse more combinations of reference dates and time deltas
more easily.

Fixes #104

[cakebaker]

Great to see you also fixed the failing test_months and test_duration_parsing in parse_relative_time.rs :)

[jfinkels]

Sorry, I don't know how to interpret the "Basic CI / Run the fuzzers" job failure. Let me know if I should do anything about that.

[jfinkels]

I'll extract some of these changes into smaller pull requests.

[jfinkels]

I'm still in the process of breaking this down into smaller changesets. A few more pull requests have been made.

[jfinkels]

Thanks for your patience. We've now merged quite a few smaller pull requests comprising changes extracted from this branch. I've now rebased on main and the smaller changeset should be easier to review now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (ca6a39c) to head (d4353d0).
Report is 4 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #110   +/-   ##
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[cakebaker]

Thanks for splitting it into multiple PRs, much appreciated :)