Added _find function to Data.List.Lens based on Data.List.find

Fixed errors documented in comments:
https://github.com/ekmett/lens/pull/228

Pervious comment:

I was reading a stackoverflow question about altering the first occurrence of an element in a list and wondered if the Lens library had such a function. Since there was none I wrote one.

Probably not the best implementation. I assume a more general implementation similar to elmenetsOf would be of more use.