RFR: 7903637: Use holder classes for string constants

[Jorn Vernee]

Use holder class idiom for string constants (See JBS issue for motivation).

The code in `emitConstant` is a bit copy-pasty. We need to strike a balance between giving a clear picture of the generated code by using a single string template as much as possible, and reducing duplication by splitting the templates up and weaving in more control flow.

I went with the repeated, but clearer approach here of having 2 separate templates. Let me know if you'd like me to change that.

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Commit messages:
 - Use holder classes for string constants
 - Share arena for all string constants

Changes: https://git.openjdk.org/jextract/pull/191/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=191&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903637
  Stats: 24 lines in 1 file changed: 13 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jextract/pull/191.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/191/head:pull/191

PR: https://git.openjdk.org/jextract/pull/191