Smaller implementations for findFirstSet, findLastSet
Smaller implementations for findFirstSet, findLastSet

Summary: [Folly] Smaller implementations for `findFirstSet`, `findLastSet`.

Reviewed By: nbronson

Differential Revision: D8471816

fbshipit-source-id: 699af98d404e6dd76df02e344d09c850abc1c32f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly