[WIP] Unified attributes and bindings

[Rich-Harris]

A realisation I've been gradually having: it makes sense to treat binding values (i.e. the foo in bind:value='foo' or bind:foo) the same way expressions are treated (i.e. {{foo}}). Doing so opens the door to a few things:

• It makes it much easier to support <Widget :foo /> as shorthand for <Widget foo='{{foo}}'/>, as mentioned in Add spread for attributes #280 (comment)
• We can more easily explore complex two-way bindings, like bind:value='options[key].value as discussed in this SO question (this is not a statement of intent at this stage, however)
• The current system messily duplicates some of the logic in generator.contextualise
• Using generator.contextualise will make sourcemap support much better

Technically this is a breaking change as it changes the AST format, but I think we can get away with it — I haven't heard of anyone using svelte.parse outside the context of svelte.compile. (I'll eat my words later if I have to.)

I've only done the first part so far — updating the parser and fixing all the tests broken by doing so. Next step is to refactor the binding code so that it uses generator.contextualise.