[foreign-memaccess] RFR 8227107: Add missing toString implementation to MemorySegmentImp

[Maurizio Cimadamore]

Hi,
this trivial fix adds a simple toString() method to MemorySegmentImpl.

This is needed in at least two places:

* MemoryAddressImpl::toString
* MemorySegmentimpl::outOfBoundsMsg

Since the toString() method can't reveal sensitive pointer info, the 
only thing we can safely expose is the segment 'limit' (e.g. the size).

http://cr.openjdk.java.net/~mcimadamore/panama/8227107/

Cheers
Maurizio