[foreign-memacces] RFR 8224993: Add Unsafe support for MemoryAddress

[Maurizio Cimadamore]

Hi,
after adding byte buffer interop support, I think it's worth looking at 
another avenue of migration - that is, translating a MemoryAddress into 
a base/offset coordinate pair, used by the Unsafe class.

http://cr.openjdk.java.net/~mcimadamore/panama/8224993/

In addition to add the trivial Unsafe capabilities, this patch also 
fixes some issues that were left unaddressed after the addition of 
ByteBuffer support - more specifically, the wrapped 'scoped' buffers do 
not have the 'address' , 'hb' and the 'capacity' fields set - 
unfortunately, these fields are accessed in a raw fashion to implement 
JNI functionalites such as GetDirectBufferAddress and 
GetDirectBufferCapacity.

I've also removed the bit-swapping from the 
VarHandleMemoryAddressAsBytes class - after all, NativeOrder deals with 
_byte_ swapping, so there's nothing meaningful we can do at the byte 
level (and that's also consistent with what ByteBuffer does).

Cheers
Maurizio