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.
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