Add a couple lemmas about product in Data.List.Properties

[dolio]

These are a couple of lemmas I found useful when trying to formalize Euclid's proof of the infinitude of primes.

• the product of non-zero naturals is non-zero
• any element of a list of non-zero naturals is less-or-equal to the product of the list

Hopefully I didn't just miss these somewhere else, but the divisibility result above this was about the only theorem I could find about product.

[jamesmckinna]

And: CHANGELOG entries please!

[jamesmckinna]

This seems fine as it is, but my nitpicks above might streamline things a bit, aesthetically at least?

[jamesmckinna]

NB. It looks as though you have forked the main repo and then done this PR directly on your master branch... it's much better (cleaner; more robust; easier to handle mutliple PRs simultaneously) to do PRs independently on separate branches off your master branch... as described in section 5 of HACKING...

[jamesmckinna]

Thanks for a very nice first PR! (NB better to fix up your git branches before the next one... ;-))
A couple of final refinements, and we should be good to go!
@JacquesCarette are you happy with the refactoring?