[foreign-memaccess+abi] RFR: Fix so that heap segments work [v2]

[Per Minborg]

> This PR fixes converting heap segments to strings.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Fix after comments

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/858/files
  - new: https://git.openjdk.org/panama-foreign/pull/858/files/2a2dfb0e..3a6e0732

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=858&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=858&range=00-01

  Stats: 9 lines in 1 file changed: 5 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/858.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/858/head:pull/858

PR: https://git.openjdk.org/panama-foreign/pull/858