RFR: 7903272: Make method handle methods private [v2]

[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.

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Revert to public method handle methods
 - Merge branch 'panama' into mh-private
 - Make method handle methods private

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/67/files
  - new: https://git.openjdk.org/jextract/pull/67/files/e4dc9cd4..3177414a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=67&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=67&range=00-01

  Stats: 157 lines in 9 files changed: 156 ins; 0 del; 1 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