[foreign-memaccess+abi] RFR: Split Stream operations, fix final and class properties [v4]

[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.

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge foreign-memaccess+abi
 - Revert some more final declarations
 - Revert most classes marked final
 - Split Stream operations, fix final and class properties

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Changes: https://git.openjdk.org/panama-foreign/pull/724/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=724&range=03
  Stats: 258 lines in 17 files changed: 74 ins; 66 del; 118 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