[foreign-abi] [Rev 01] 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

The pull request has been updated with 1 additional commit.

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Added commits:
 - 464a70f4: * tweaked MemoryAddress::ofLong to return NULL if value is zero

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/19/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/19/files/c70a8aeb..464a70f4

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/19/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/19/webrev.00-01

  Stats: 16 lines in 2 files changed: 2 ins; 7 del; 7 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