[foreign-memaccess+abi] RFR: Modernize code [v5]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'foreign-memaccess+abi' into modernize-code
 - Remove the use of Unsafe
 - Rename and reformat
 - Use allocation free constructs
 - Improve switch and use record for return type
 - Modernize code

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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/7c758bd6..fd988e39

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=04
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=03-04

  Stats: 18 lines in 13 files changed: 0 ins; 0 del; 18 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