Components in each-blocks with two-way binding don't always update

[Rich-Harris]

This works...

{{#each a as b}}
	<Widget value='{{b}}'/>
{{/each}}

...but this doesn't:

{{#each a as b}}
	<Widget bind:value='b'/>
{{/each}}

Reason is the discrepancy between this code and this code — in the first case Svelte is checking if the dependencies of b (i.e. ['a']) are among the changed data properties, in the second case it's checking if b itself is among them (which of course it isn't).