By being a bit less greedy with consuming whitespace in the date/time
parsers, we can make the `prop_{encode,decode}Preservation` properties
well-defined again.
Add the trivial Arbitrary instance for Priority, extend the Arbitrary
instance of OrgMsg, as well as some plumbing. Also work in some unit
tests for regression testing.
Ever since [1] we allow a second representation for the month (namely,
the numerical one). Since we lose this information during parsing,
pretty printing is now not a proper postinverse of parsing (it still is
a proper preinverse, however).
Thus, we can't simply check for an inverse anymore. However, the
operations still form an inverse semigroup [2], which is something
that's easily checkable. For simplicity, do this in both directions and
completely forget about linearity for now.
[1]: 91f1a0de1e4a536e6c81a3de96e566622b3eb20a (Fix date parsing issue
for org mode plugin)
[2]: https://en.wikipedia.org/wiki/Inverse_semigroup
This patch fixes the date parsing issue currently when an entry like
`todo +d 22 01 2022` is used. I have added tests too which demonstrate
the current issue so that we can prevent future regression.
