[foreign-memaccess+abi] RFR: Modernize code [v2]

[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:

  Improve switch and use record for return type

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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/a12d1817..9b7e1fa6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=00-01

  Stats: 36 lines in 1 file changed: 0 ins; 13 del; 23 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