Fix order of oncreate in sibling components

[Rich-Harris]

This fixes #1135 by running oncreate hooks (and associated beforecreate and aftercreate) front to back instead of back to front. We have to tweak the initialisation code to preserve the depth-first behaviour, which people might be relying on (there's a test that says as much).

This does change one piece of behaviour in a way that might be unexpected — if you have uninitialised bindings like this...

{{#if x}}
  <Foo bind:y/>
{{else}}
  <Bar bind:y/>
{{/if}}

...the binding of y will initialise to the opposite value than what it did before. Uninitialised bindings inside conditionals is a terrible idea that is easily fixed by the user, so I don't feel bad about that.