Revert "Preventing SimpleValue from nesting, with associated ValueProvider A

This passes the ftests we exercise, but breaks a lot of things for real use.
They rely on $A.get("{!$Label...}"), so it's still using the getValue API, a
by unwrapping the simpleValue in its assignment, things get wonky.

This reverts commit fb3e79cd4cc56e4ff0488e672391a2de56a510f1.
Merging origin/fabbott/master-pending-revert-unnest into master