[foreign-memaccess+abi] RFR: Remove dead code, add tests and fixes [v3]

[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 one additional commit since the last revision:

  Cleanup after comments

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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/02d7a061..4f111140

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=792&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=792&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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