add ScalaCheck-based ULong test
add ScalaCheck-based ULong test

tixxit/spire,siddhartha-gadgil/spire,codeaudit/spire,adampingel/spire,AlecZorab/spire,JackSullivan/spire,rklaehn/spire,kevinmeredith/spire,codeaudit/spire,guersam/spire,guersam/spire,non/spire,woparry/spire,non/spire,rklaehn/spire,siddhartha-gadgil/spire,AlecZorab/spire,tixxit/spire,woparry/spire,kevinmeredith/spire,adampingel/spire,JackSullivan/spire