Slot-based routing

[Rich-Harris]

ref #573. Mostly done, but a bunch of tests that aren't passing yet

[Rich-Harris]

Been dogfooding this at work all day and I do prefer it — much less paranoid about state change false positives causing unwanted side-effects. That said, I kind of miss being able to pass props from a layout component to its children, so that /user/alice/friends and /user/alice/enemies can both share an object representing Alice.

It's definitely solvable with the context API, but not quite as elegant. One possible solution would be to have a way to expose all slot props:

-<svelte:component this={level1.component} segment={segments[1]} {...level1.props}>
+<svelte:component this={level1.component} segment={segments[1]} {...level1.props} let:*={props}>
  {#if level2}
-    <svelte:component this={level2.component} {...level2.props}/>
+    <svelte:component this={level2.component} {...props} {...level2.props}/>
  {/if}
</svelte:component>

Can cross that bridge another time though.