Add 'zoomMaybe'

[paf31]

This seems like it would be useful for applications with multiple tabs.

[zrho]

I think we are even better off if we make zoomAll produce an element in some Plus functor, by folding the resulting list with <|> and empty. This would include the list type as currently there, but also arrays and maybe. That would not check the traversal to be a prism, but in the case it is not, it degrades gracefully by only picking the first target of the traversal.

[paf31]

Sounds much better.