store.onchange.cancel() function not scoped correctly.

[GarethOates]

When attempting to cancel the onchange event listener on my svelte store.

let handler = store.onchange(storeChanged);
handler.cancel();

I kept receiving the following error:

TypeError: this._changeHandlers is undefined

This is due to the scope of the returned object which contains the cancel() function not having a reference to the changeHandlers array.

The workaround in code that I did for this was the following, which I suspect is not what you want the user to have to do.

handler.cancel.bind(store)()

I've written a pull request to fix this, which binds the cancel() function to this, in order to maintain the reference to this._changeHandlers.

return {
    cancel: function() {
		var index = this._changeHandlers.indexOf(callback);
		if (~index) this._changeHandlers.splice(index, 1);
    }.bind(this)
};

[Rich-Harris]

Released 1.55.1 with the fix, thank you!