Add find operation to FoldableOps

[bilki]

This adds the find operation to FoldableOps, to match the first element of the structure for which the partial function passed as argument is defined.

I'm not sure if this is the correct semantic of find operation for a given R recursive type. I have another implementation taking a boolean predicate like any, or all operations do, but then the result type of find changes from Option[B] to Option[R], returning the whole substructure from where the first match succeeded.

I also had to add a new tag First and the Option monoid in order to use foldMap. Another concern is that I don't know if this shortcircuits on the first match (does any operation shortcircuit?).

Any feedback is more than welcome, as this is my first time working with recursion schemes concepts.

[andyscott]

I'd prefer the Boolean variation, as it's more consistent with find methods in the Scala stdlib.