add Lens for Map.WithDefault

For when you don't need a PLens.
standalone Set membership lens

Similar to SetLensFamily.contains, but doesn't require input Lens.
Change Scalacheck URL to use Scaladoc syntax so generated HTML is valid
Implement traverseU_
Merge pull request #547 from refried/patch-10

add Lens for Map.WithDefault
Merge pull request #548

standalone Set membership lens
Merge pull request #556 from adelbertc/fix-scalacheck-url

Change Scalacheck URL to use Scaladoc syntax so generated HTML is valid
Merge pull request #558 from bmjames/traverseU_

Unapply version of traverse_