Add mutable access to first and last

[pteasima]

Hey pointfreeguys,

I sometimes want to modify the first or last element of a NonEmpty collection. Afaik Swift.MutableCollection doesn't allow this because of the optionality (semantics of setting nil would be unclear). However, NonEmpty doesn't have this problem.

Im currently using the below proxy, which achieves someNonEmptyCollection.mutable.last = ... syntax.

However, I think it should be pretty easy to add a setter directly to first and last properties to achieve someNonEmptyCollection.last = ... syntax. I personally don't see many downsides, but I feel like you might shut this down on the grounds of deviating from Swift standard library. So I wanted to get a quick 👍 or 👎 before I start working on a PR.

Thanks!