Consider supporting address dereference/allocation for platforms where pointers are not 64 bits

MemoryAccess make assumptions on the pointer size - e.g. it lets clients dereference a memory address, but assuming that the underlying value is 8 bytes.

On the other hand, NativeScope refuses to make such an assumption and does not provide any allocation helper to work on addresses.

The situation seems inconsistent - but, unless we also decide to drop address dereference operation from MemoryAccess, we should find a way to drop the hardwired constraint that pointers are represented as 64 bit values.