Add array destructuring for each contexts

[jacobmischka]

Example repo, ran with svelte npm linked to branch as indicated in readme.

Note this does not do object destructuring, just array destructuring.

Input:

<div>
	{{#each Object.entries(map) as [key, value], i}}
		<h2>{{ key }}: {{ i }}</h2>
		{{#each Object.entries(value) as [subkey, subvalue], j}}
			<p>
				{{subkey}}: {{subvalue}}
			</p>
		{{/each}}
	{{/each}}
</div>

<script>
export default {
	data() {
		return {
			map: {
				key: {
					subkey: 'subval',
					subkey2: 'subval2'
				},
				anotherKey: {
					anotherSubkey: 'anotherSubval',
					anotherSubkey2: 'anotherSubval2'
				}
			}
		};
	}
};
</script>

Result:

Fixes #888

[jacobmischka]

Oops hang on I forgot some tests and missed some ssr bugs, will reopen when fixed

[jacobmischka]

Okay looks like CI doesn't like Object.entries in the test, fair enough.

[Rich-Harris]

Nice, thank you!