RFR: 7903626: Add struct helper method to reintrepret an array of structs

[Jorn Vernee]

This patch renames the existing `ofAddress` method we generate in struct classes to `reinterpret` to reflect the latest naming changes in the FFM API. It also adds another overload that takes an element count, for reinterpreting arrays. Finally, this patch adds the missing cleanup parameter that the underlying reinterpret method accepts.

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Commit messages:
 - add reinterpret helper

Changes: https://git.openjdk.org/jextract/pull/178/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=178&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903626
  Stats: 229 lines in 9 files changed: 126 ins; 97 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/178.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/178/head:pull/178

PR: https://git.openjdk.org/jextract/pull/178