Ported ExecutionEngine::newString and newIdentifier to Heap::String

Avoid the use of Returned<String> for newString and changed the identifier
table to use Heap::String. This required moving some code back into
Heap::String, but that's code that doesn't call back into the GC, so
allocations and therefore future object moves aren't possible.

Change-Id: I1dca3e9c12a9c56f09419af8cc8cba39fe04f720
Reviewed-by: Lars Knoll <lars.knoll@digia.com>