[WIP] Separate unmount from destroy

[Rich-Harris]

#592 revealed a slight flaw in how Svelte works — unmounting blocks (removing them from the DOM) and destroying them (removing event listeners etc) are coupled, when they actually serve separate purposes. In the case of #592, where a yield block exists inside an if block, we want to unmount the yield fragment when the if condition becomes falsy, not destroy it.

This PR separates the two things, so that blocks have unmount and destroy methods. fragment.destroy therefore no longer takes a detach argument that determines whether or not to unmount.

Generated code size has increased slightly as a result of this change, but as a follow-up we can determine during the preprocess step whether or not a given block needs a destroy method (i.e. does it contain event handlers, components etc, or does unmount take care of cleanup?) and omit it if possible, the same way we do with update methods.

[Rich-Harris]

Should mention: this is most likely a breaking change in the edge case where your imported components were compiled with an old version of Svelte, a la #586.

#604 contains a proposal for handling this situation better at a more general level.