Avoid binding event handler callbacks, version 2

[Rich-Harris]

#449, but on top of #453. Got a bit sidetracked into refactoring the Element visitor code, so haven't actually started on the event handler stuff per se, but that'll come soon. Element stuff is starting to feel a bit more logical and easier to follow.

[Rich-Harris]

Taking the WIP label off this now. Event handlers are hoisted to the top level if a) they're not custom events (which can't be hoisted), and b) they're inside an each block. This means that in a situation like this...

{{#each veryLongList as item}}
  <button on:click='set({ selected: item })'>{{item}}</button>
{{/each}}

...the click handler only needs to be declared once, leading to lower memory use and better performance.

Custom event handlers are no longer bound — because they're declared inside a block where they have access to the node in question, references to this can simply be rewritten.