RFR: 7903597: Slim down RuntimeHelper [v2]

[Jorn Vernee]

> Remove some of the helper methods in RuntimeHelper. They can be moved inline in the generated code.
> 
> Also: I've elected to move the `System.load(Library)` directives we generated inside RuntimeHelper into the first header class, in order to have it more localized to the actual code that a user sees. That also makes RuntimeHelper more 'dumb'. Only the package declaration is now injected into the template, which makes it easier for users to combined multiple extraction runs (they can just discard one of the generated RuntimeHelpers).

Jorn Vernee has updated the pull request incrementally with two additional commits since the last revision:

 - add back downcallHandleVariadic
 - remove RuntimeHelper. Emit helper methods in first header instead

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/150/files
  - new: https://git.openjdk.org/jextract/pull/150/files/b8543a05..d229455b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=150&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=150&range=00-01

  Stats: 209 lines in 11 files changed: 80 ins; 91 del; 38 mod
  Patch: https://git.openjdk.org/jextract/pull/150.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/150/head:pull/150

PR: https://git.openjdk.org/jextract/pull/150