Optimizations

Some code optimizations:

• Several methods updated to use raw for() loops instead of Array.prototype.forEach and Array.prototype.map.
• Changed "isKey()" method to use Object.hasOwnProperty() on the original enum object instead of relying on a ES6 Set of keys.
• Use Array.prototype.slice to return defensive copies of lists of keys and values (instead of mapping from list of entries)
• Keys are now stored as a plain array instead of an ES6 Set.