[foreign-memaccess] RFR 8226526: Add extra static checks to MemoryAccessVarHandles

[Maurizio Cimadamore]

Hi,
as discussed in [1], here's a patch which adds extra checks to the 
combinator methods in MemoryAccessVarHandles.

This also consolidates the combinators API, by removing the alignment 
and byte order combinators (which are now part of the leaf combinator 
method).

I also moved MemoryAccessVarHandle from j.l.invoke to java.foreign - we 
want to keep the foreign classes close together in preparation for when 
we'll need to push them onto a different (incubating) module.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8226526/

Cheers
Maurizio