[foreign-memaccess+abi] RFR: Remove dead code, add tests and fixes [v2]

[Per Minborg]

> This PR suggests removing dead code and addition of some tests. Also, a fix for zero-length mapped segment is added. It is anticipated this special zero-length segment is going to be removed in a separate PR.

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Fix copyright years
 - Remove zero-length fixes and more

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/792/files
  - new: https://git.openjdk.org/panama-foreign/pull/792/files/742f8afc..02d7a061

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=792&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=792&range=00-01

  Stats: 77 lines in 5 files changed: 34 ins; 37 del; 6 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/792.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/792/head:pull/792

PR: https://git.openjdk.org/panama-foreign/pull/792