[foreign-memaccess] RFR 8225172: Add way to create deref VarHandles without using the Layout API

[Jorn Vernee]

Hi,

I've implemented an alternative way of creating memory access 
VarHandles, which bypasses the Layout API. One reason for having such an 
alternative API is that it offers users the opportunity to use their own 
(pre-existing) layout APIs when creating VarHandles. This mainly 
consists of a builder API called MemoryAccessVarHandleBuilder, which can 
be used similarly to LayoutPath to construct a VarHandle.

As a side effect I've removed the dependency on Layout/LayoutPath from 
VarHandles::makeMemoryAddressViewHandle. I think this decoupling makes 
this API point more suited to what's possible to do with memory access 
VarHandles.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225172
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8225172/webrev.01/

Added tests are pretty minimal since the resulting VarHandle is already 
tested by the existing tests using the Layout API.

This also opens up the possibility of removing the Layout API altogether 
if we wanted. Another TODO is adding a way to disable alignment checking 
upon access (which would need changes to the VarHandle impl classes I 
think).

Thanks,
Jorn