RFR: 7903625: Simplify API of the generated code

[Jorn Vernee]

On Fri, 19 Jan 2024 12:16:27 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> 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).

This looks good. Could you add a test case for the new setter as well?

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Marked as reviewed by jvernee (Committer).

PR Review: https://git.openjdk.org/jextract/pull/187#pullrequestreview-1832692262