[foreign-memaccess+abi] RFR: Split Stream operations, fix final and class properties [v2]

[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 incrementally with one additional commit since the last revision:

  Revert most classes marked final

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/724/files
  - new: https://git.openjdk.org/panama-foreign/pull/724/files/5932213a..2d6cf5cd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=724&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=724&range=00-01

  Stats: 16 lines in 13 files changed: 0 ins; 0 del; 16 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