Adds property tests in both directions for the parser in X.U.EZConfig.
As with the tests for X.P.OrgMode, these (modulo `Maybe` noise) take the
shape of the operation of an inverse semigroup:
pp ∘ p ∘ pp ≡ pp and p ∘ pp ∘ p = p,
where pp is pretty-printing and p is parsing.
When users specify non-existent keys, it seems most intuitive to just
abort the parse and not try to take the "longest" input that still
works. For example, given the "key" `M-10` we should signal a parse
error (by returning `Nothing`) instead of parsing `M-1` and ignoring
the rest of the input. The old EZConfig parser accounted for this but
when the module was rewritten to use X.U.Parser in [1], this was
forgotten about.
Fixes: https://github.com/xmonad/xmonad/issues/361
[1]: 8abeb81fd0693bd4ee914b522c0a2a2cfcfaf0dd
Add very basic unit tests for EZConfig to see if it can parse all of the
keys (and key combinations) that it promises to parse.
The long-term goal here should be to write a pretty-printer for EZConfig
and to check whether that's a proper inverse (either in the normal sense
or in the inverse semigroup sense), as the tests for X.P.OrgMode do.
