cleanup: Get rid of Lang.bind()
After replacing Lang.Class with ES6 classes and adopting arrow notation
for anonymous callbacks, we only use the Lang module to bind this to
named callbacks. However since ES5, this functionality is already provided
by Function.prototype.bind() - in fact, Lang.bind() itself uses it when
no extra arguments are specified. Just use the built-in function directly
where possible, and use arrow notation in the few places where we pass
additional arguments.