[foreign-memaccess+abi] RFR: Change classes into records and fix finality

[Per Minborg]

This PR converts some of the regular classes into records. It also fixes some finality issues.

Note: Some methods with a very long method parameter list have their parameter list broken up into separate lines.

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Commit messages:
 - Change classes into records and fix finality

Changes: https://git.openjdk.org/panama-foreign/pull/725/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=725&range=00
  Stats: 298 lines in 29 files changed: 53 ins; 85 del; 160 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/725.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/725/head:pull/725

PR: https://git.openjdk.org/panama-foreign/pull/725