RFR: Make some classes/fields final and add privat ctr

[Per Minborg]

This PR makes some of the classes and fields `final`. Also, a general cleanup of the `RuntimeHelper` class is proposed.

There are some files that are "overhidden" (by attempting to "override"/hide static methods). This will be fixed in a separate PR and so these classes are not affected in this PR.

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Commit messages:
 - Make some classes/fields final and add privat ctr

Changes: https://git.openjdk.org/jextract/pull/65/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=65&range=00
  Stats: 99 lines in 6 files changed: 49 ins; 19 del; 31 mod
  Patch: https://git.openjdk.org/jextract/pull/65.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/65/head:pull/65

PR: https://git.openjdk.org/jextract/pull/65