Only re-iterate when the iterable has changed
Only re-iterate when the iterable has changed

(Finishing the remaining work from #227)
chadhietala/glimmer,lbdm44/glimmer-vm,glimmerjs/glimmer-vm,chadhietala/glimmer,glimmerjs/glimmer-vm,chadhietala/glimmer,glimmerjs/glimmer-vm,lbdm44/glimmer-vm,tildeio/glimmer,tildeio/glimmer,tildeio/glimmer,lbdm44/glimmer-vm,chadhietala/glimmer