RFR: 7903625: Simplify API of the generated code

[Maurizio Cimadamore]

This PR simplifies the API of the code generated by jextract. Here's a list of the changes implemented:

* the struct layout accessor is now just named `layout()`
* functional pointer code generation is different now: we generate a toplevel _class_ which defines a nested functional interface that is used for the factory method. This means that we can hide some of the constants associated with the function pointer. The descriptor accessor remains, but is called `descriptor()`.
* For global variables, accessors such as `x$SEGMENT` and `x$LAYOUT` were made `private`
* As for struct fields, I've made the accessors for global variable work for the case where the global is a struct/array too (the getter/setter will work on a MemorySegment).

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Commit messages:
 - Merge branch 'panama' into cleanup_generated_api
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/187/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=187&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903625
  Stats: 216 lines in 33 files changed: 53 ins; 28 del; 135 mod
  Patch: https://git.openjdk.org/jextract/pull/187.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/187/head:pull/187

PR: https://git.openjdk.org/jextract/pull/187