[foreign-memaccess+abi] RFR: Modernize code [v4]

[Per Minborg]

> This PR proposes to modernize some parts of the code with constructs from newer Java versions.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove the use of Unsafe

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/894/files
  - new: https://git.openjdk.org/panama-foreign/pull/894/files/14d54487..7c758bd6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=03
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=02-03

  Stats: 102 lines in 3 files changed: 33 ins; 35 del; 34 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/894.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/894/head:pull/894

PR: https://git.openjdk.org/panama-foreign/pull/894