RFR 8238192: Reimplement MemoryAddress memory access handles on top of var handle combinators

[Maurizio Cimadamore]

Hi,
this is another use case for VarHandle combinators - we have added 
support for VarHandles whose carrier is a MemoryAddress - but the way we 
did this was to spin dedicated var handle classes for this. Since we now 
have the ability to adapt VarHandle we no longer need to spin dedicated 
code, so we can revert much of the code and implement 
MemoryAddress-bearing VarHandles as adapters, on top.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8238192/

P.S.
I couldn't quite simplify the code as much as I'd have liked - because 
we support another kind of combinators in the MemoryHandles class which 
take apart an existing memory access handle and "repackage it" with 
different coordinates. These combinators will be eventually superseded 
by the official VarHandle combinators, so they will go away in the long 
term - but for now there is a concern that removing those combinators 
might be too soon (given that performances of an adapter using 
MemoryAddress::addOffset will be worse than what we have with 
MemoryHandles.withOffset).

Maurizio