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

[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

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Commit messages:
 - Merge branch 'foreign-memaccess+abi' into cleanup_allocator
 - Drop use of cleaner in valists
 - Cleanup javadoc for ResourceScope/SegmentAllocator
 - Minore javadoc tweaks
 - Revert some API examples to use MemorySegment.allocateNative
 - Fix javadoc summary for MemoryLayout#isPadding
 - Cleanup javadoc
 - Revert more uses of newConfinedScope to newImplicitScope
 - Merge branch 'foreign-memaccess+abi' into cleanup_allocator
 - Revert polarity of scope factories
 - ... and 6 more: https://git.openjdk.java.net/panama-foreign/compare/3882cde6...2976e33b

Changes: https://git.openjdk.java.net/panama-foreign/pull/591/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=591&range=00
  Stats: 788 lines in 59 files changed: 148 ins; 283 del; 357 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