Cleanup and unify context creation

Cleanup and unify context creation

* Always create contexts on the heap. When embedding native methods
we can't create contexts on the stack anymore without running the
risk of them being used in some scope chain.
* Unify context creation for call contexts, share the code
* Add a hack for indirect calls to eval and create a new 'fake'
global context there, so we don't mess up the context stack (it
broke badly when unwinding exceptions before).

Change-Id: I5804224dc26582f24ec79518639ceb13a8a3e967
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>