Optimisation — only bind custom event callbacks if necessary

[Rich-Harris]

Follow-up to #429. Binding callbacks is cheap, but not free, so we should avoid it if we can.

In this example...

<button on:longpress='set({ count: count + 1 })'>+1</button>

...the callback needs to be bound, because the event handler needs a reference to root for count. In this example...

<button on:longpress='set({ count: 0 })'>reset</button>

...it doesn't.

In fact, since the event handler is necessarily added inside a closure where component is available, maybe it should only be bound if this is used, and the handler should be augmented like so:

// this...
var longpressHandler = template.events.longpress.call( component, button, function ( event ) {
  var root = component.get();
  component.set({ count: root.count + 1 });
});

// rather than this:
var longpressHandler = template.events.longpress.call( component, button, function ( event ) {
  var root = this.__svelte.root;
  component.set({ count: root.count + 1 });
});

That would obviate the button.__svelte reference altogether, allowing us to get rid of some code (though you'd still need something analogous to track the index in each blocks in situations like this).

[Rich-Harris]

maybe it should only be bound if this is used

Or we could rewrite this as button (or whatever the node's variable name is). Then there's no binding necessary