[foreign-memaccess] RFR: Investigate ways to make handoff-like operation more explicit (foreign-memacess edition)

[Maurizio Cimadamore]

This is a subset of the patch proposed here:
https://git.openjdk.java.net/panama-foreign/pull/361

I'd like to integrate the memory access bits first and then merge the changes into PR 361.

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Commit messages:
 - Revamp handoff functionalities and javadoc

Changes: https://git.openjdk.java.net/panama-foreign/pull/363/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=363&range=00
  Stats: 611 lines in 18 files changed: 213 ins; 284 del; 114 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/363.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/363/head:pull/363

PR: https://git.openjdk.java.net/panama-foreign/pull/363