Replaced __proto__ with Object.setPrototypeOf()

Table.mapToClass() and Table.defineClass() would set __proto__ to make
objects derive from given class. According to MDN documentation,
__proto__ is not only deprecated but will also slow down the javascript
engine. By a jsfiddle test, it turned out that most browsers (including
IE11) now support Object.setPrototypeOf() so I have changed to using
that one instead. Still fallbacks to object cloning for older browsers
(like IE10).