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.
