[foreign-memaccess] RFR: JDK-8243300: Move var handle combinators into foreign incubator module

[Maurizio Cimadamore]

This patch moves all the var handle combinators back into the incubator module, so that no change in the public API
occurs. To do this, I had to split the combinator implementation (which has been moved into j.l.i.VarHandles) from the
public API points, which are in jdk.incubator.foreign.MemoryHandles; the latter can see the former through the usual
JLI access.

-------------

Commit messages:
 - Move var handle adapters from MethodHandles into incubator module (MemoryHandles)

Changes: https://git.openjdk.java.net/panama-foreign/pull/123/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/123/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8243300
  Stats: 972 lines in 8 files changed: 505 ins; 407 del; 60 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/123.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/123/head:pull/123

PR: https://git.openjdk.java.net/panama-foreign/pull/123