Document and test laws for Monus.

[jonathanknowles]

This PR adds explicit documentation and tests for the following Monus laws:

a <\> a == mempty
mempty <\> a == mempty
a <> (b <\> a) == b <> (a <\> b)
(a <\> b) <\> c == a <\> (b <> c)

These are equivalent to the laws stated here.

Furthermore, this PR tests that the following equivalences hold:

(a <\> b) == stripPrefixOverlap b a
(a <\> b) == stripSuffixOverlap b a

Prior to this PR, it looks like the existing Monus instances were not explicitly tested. (The Test data type did already have a MonusTest constructor, but this wasn't used in the definition of tests, and the import of <\> was redundant.)

Finally, this PR adds test coverage for all instances of Monus provided by the library.

[jonathanknowles]

Hi @blamario

Hope it's okay for me to create this PR. Of course, let me know if you'd like me to change anything!

[blamario]

Thank you! I don't know how I missed the monus laws on Wikipedia.