Change WeakMap hirerarchy

The library currently accesses by function, and then by context. It's much more likely that the context will be garbage collected than a function.

eg:

function Foo {
  this._message = 'hi';
}

Foo.prototype.bar = function () {
  console.log(this._message);
}

var foo = new Foo();

bind(foo.bar, foo);

As you can see, with OOP, foo.bar, which is really Foo.prototype.bar, will never be garbage collected, however the foo instance will be.

By reversing the WeakMaps, it's very likely that the unbind function isn't needed anymore, making idempotent-bind a true drop-in replacement.

I can submit a PR if this change is welcome.

azu / idempotent-bind