[foreign-memaccess+abi] RFR: API refresh - part two [v6]

[Maurizio Cimadamore]

> After playing a bit more with the new API changes, I realized there were issues with some of the solutions proposed in [1].
> I will add more details in a separate comment, to keep the header of this PR compact.
> 
> [1] - https://git.openjdk.java.net/panama-foreign/pull/576

Maurizio Cimadamore has updated the pull request incrementally with three additional commits since the last revision:

 - Fix TestByteBuffer to use implicit scope on some negative test
   Revert CallOverheadHelper to use confined/shared points
 - Merge branch 'cleanup_allocator' of https://github.com/mcimadamore/panama-foreign into cleanup_allocator
 - Fix javadoc issues

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/591/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/591/files/c530e3f4..f17ac3d6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=591&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=591&range=04-05

  Stats: 14 lines in 4 files changed: 0 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/591.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/591/head:pull/591

PR: https://git.openjdk.java.net/panama-foreign/pull/591