[WIP] Avoid binding event handler callbacks

[Rich-Harris]

Ref #433. Before starting on this, I figured it'd make sense to add a new category of tests, so we can verify the code generation is working as expected. We should avoid making too many of those tests since keeping them current could become a chore, but I don't see a good alternative for stuff like this.

[Rich-Harris]

This is still WIP — still relies on this for getting contextual data, which apparently there aren't any tests for (otherwise I would expect this to fail). Realised that it probably makes sense to work on hoisting normal event handlers first (it makes no sense to recreate handlers for items in a list, when we can just stick contextual info on node.__svelte and share a handler between multiple listeners), and that puts a bit more pressure on refactoring some of the generator/fragment stuff, so I think I'm going to put this on pause for a bit and investigate that.

[Rich-Harris]

closing in favour of #456