[foreign-memaccess] RFR: Memory access implementation rewrite - post cleanup

[Maurizio Cimadamore]

* Fix access modes not preserved in AbstractMemorySegmentImpl.ofBuffer
* Add check to ensure that Foreign::withSize and Foreign::asMallocSegment are called with unchecked address

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Commit messages:
 - * Fix access modes not preserved in AbstractMemorySegmentImpl.ofBuffer

Changes: https://git.openjdk.java.net/panama-foreign/pull/111/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/111/webrev.00
  Stats: 54 lines in 7 files changed: 43 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/111.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/111/head:pull/111

PR: https://git.openjdk.java.net/panama-foreign/pull/111