each-block array destructuring

[MikeMatrix]

Inspired by #2647, I've been messing about with the parser code to integrate array destructuring.
Might be of use for somebody working on that issue.

This should allow something like:

{#each sequence as [first, ...rest]}
   <b>{first}</b><i>{rest}</i>
{/each}

[Rich-Harris]

this is very cool, thank you!