Fix run-time string handling with regards to the new heap

Changed runtimeStrings to be an array of Heap::String pointers instead of
indirect String pointers. Later that member along with other GC related members
will go into a managed subclass. Meanwhile the generated code no more loads
String pointers directly but just passes the index into the run-time strings to
the run-time functions, which in turn will load the heap string into a scoped
string.

Also replaced the template<T> Value::operator=(T *m) with a non-template
overload that takes a Managed *, in order to help the compiler choose the
non-template operator=(Heap::Base *) overload. This allows removing a bunch
of Value::fromHeapObject calls.

Change-Id: I20415c0549d33cca6813441a2495976b66d4c00e
Reviewed-by: Lars Knoll <lars.knoll@digia.com>