[foreign-memaccess+abi] RFR: Split Stream operations, fix final and class properties

[Per Minborg]

This PR splits Stream operations into separate lines, and fixes some `final` and class issues.

Some smaller issues are also fixed not related to the above.

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Commit messages:
 - Split Stream operations, fix final and class properties

Changes: https://git.openjdk.org/panama-foreign/pull/724/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=724&range=00
  Stats: 190 lines in 25 files changed: 43 ins; 66 del; 81 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/724.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/724/head:pull/724

PR: https://git.openjdk.org/panama-foreign/pull/724