Delivers significant performance improvements by caching previously computed results.

# Conflicts:
#	edtf/fields.py
#	edtf/jdutil.py
#	edtf/natlang/en.py
#	edtf/natlang/tests.py
#	edtf/parser/grammar.py
#	edtf/parser/parser_classes.py
#	edtf/parser/tests.py
#	pyproject.toml
#	setup.py

Also added Andrew Hankinson to the authors list in pyproject.toml

I've had a pass at the Parser Classes file, but there are a lot of problems still to be sorted out.

I've added return types and argument types whereever it makes sense.

The "UncertainOrApproximate" class is a hot mess. There are boolean values with property and method calls associated with them, and I would be surprised if it actually works. However, it doesn't seem to be tested or implemented, so I can't figure out where to go from here.

This wasn't actually used anywhere!

Also removed a redundant regex group