[foreign-memaccess] RFR 8224483: Split MemoryAddress into separate address/region abstractions

[Maurizio Cimadamore]

Following discussion at [1], I decided to go ahead and split 
MemoryAddres into two abstractions:

* MemroyAddress will now embody an offest into a...
* MemorySegment, which represent a contiguous region of memory

The name MemorySegment came from an internal discussion with Brian, 
where he pointed out that MemoryScope and MemoryRegion where a bit too 
overlapping, in the english sense of the word. MemorySegment suggests 
something that has lower/upper boundaries.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8224483/

There are many changes since the last patch discussed - mostly because, 
as I was adding the lower-level allocate methods in MemoryScope I 
realized that we were missing a lot of checks (w.r.t. alignment and 
sizes). I've added those and added tests for these as well (2 new tests 
have been added). Also, the Scope::allocate API should reflect the fact 
that Unsafe might fail to allocate - again added a test for this.

Maurizio