Separate/refactor forward pointer access

Accessing the forward pointer overlay of object headers is currently done in a somewhat crude way. oopDesc::forwardee() and oopeDesc::is_forwarded() both load the header-word. This seems kinda bad, because they are most often used in conjunction, e.g.:
fwd = obj->forwarded() ? obj->forwardee() : promote_obj();

or similar constructs.

Also, in some places, the forwardee is accessed in a more direct fashion using markWord::decode_pointer(), or as HeapWord*, sometimes is_forwarded() is determined by forwardee() == NULL checks (and crude overrides to prepare non-forwarded headers to return NULL in such checks, see g1Full* source files).

I propose to extract and refactor forward pointer access in a way to better support above pattern. In-fact I originally wrote this with performance-enhancement in mind (see oopForwarding.hpp header for more details), but I could not show any actual performance improvements, so let's consider this purely on cleanliness and separation-of-concerns basis.