RFR: 7903272: Make method handle methods private

[Per Minborg]

xxxx$MH methods should be declared private to reduce cluttering in end users' IDEs and elsewhere.
 
 This PR also solves esthetic issues in the generated code.

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Commit messages:
 - Make method handle methods private

Changes: https://git.openjdk.org/jextract/pull/67/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=67&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903272
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/67.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/67/head:pull/67

PR: https://git.openjdk.org/jextract/pull/67