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

[Maurizio Cimadamore]

This is a continuation of the RFR here:
https://mail.openjdk.java.net/pipermail/panama-dev/2020-January/007366.html

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Commits:
 - c70a8aeb: 8238192: Reimplement MemoryAddress memory access handles on top of var handle combinators

Changes: https://git.openjdk.java.net/panama-foreign/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/19/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8238192
  Stats: 120 lines in 5 files changed: 24 ins; 85 del; 11 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/19/head:pull/19

PR: https://git.openjdk.java.net/panama-foreign/pull/19