[foreign-jextract] RFR: 8246400: jextract should generate a utility to manage mutliple MemorySegments

[Athijegannathan Sundararajan]

* Added RuntimeHelper.Scope class.
* Piggybacking to fix test description and unused imports in another test.

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Commit messages:
 - 8246400: jextract should generate a utility to manage mutliple MemorySegments

Changes: https://git.openjdk.java.net/panama-foreign/pull/192/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/192/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246400
  Stats: 205 lines in 5 files changed: 202 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/192.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/192/head:pull/192

PR: https://git.openjdk.java.net/panama-foreign/pull/192