[foreign-memaccess] RFR 8235837: Memory access API refinements

[Maurizio Cimadamore]

Hi,
following the discussion on the API refinements in [1], we have 
converged on the two changes listed below:

* drop the MemoryAddress::offset()
* add an overload of MemorySegment::asSlice to create a new slice given 
a new memory address (base) + length
* replace isAccessible with a owner() accessor, which returns the owning 
thread

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8235837/

Cheers
Maurizio