Add Monoid/Semigroup for Forget

[ChrisPenner]

This hopefully isn't too controversial since this is the natural Monoid for the underlying function; in fact you could deriving via (a -> r) if you wanted.

Adding this allows us to smash together Folds with profunctor optics; e.g. you get (x ^.. to f <> to g) == [f x, g x] which is nice, and this instance just generally does the Right Thing™ when working with Forget 😄

Let me know if there's anything else you need 👍

Thanks for maintaining this library; it's taught me a lot over the years!

[ChrisPenner]

Sounds good; I've addressed everything 👍