RFR: 7903597: Slim down RuntimeHelper

[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).

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Commit messages:
 - remove LINKER field
 - rename emitLoadLibrary
 - Remove requireNonNull & CONSTANT_ALLOCATOR
 - drop upcallStub helper
 - remove spurious imports
 - clean up test name
 - slim down RuntimeHelper

Changes: https://git.openjdk.org/jextract/pull/150/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=150&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903597
  Stats: 298 lines in 9 files changed: 97 ins; 177 del; 24 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