RFR: 7903947: Access to function pointers in structs could be streamlined

[Nizar Benalla]

Please review this patch to add a new option where you can generate an direct invoker methods. This feature is intended to be used on structs that are just a collection of functions we had like to call.

If this feature is used, we remove the getter to avoid name collisions as we assume this is similar to an interface.

Changes to `GUIDE.md` have been intentionally left out from this initial patch to make reviews easier.


struct Foo {
    struct Bar (*a)(void);
    struct Bar (*b)(int);
};


Before 


var foo = alloc_callback_h.foo();

var barA = Foo.a.invoke(Foo.a(foo), arena);
var barB = Foo.b.invoke(Foo.b(foo), arena, 100);


After 


var foo = alloc_callback_h.foo();

var barA = Foo.a(foo, arena);
var barB = Foo.b(foo, arena, 100);

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

Commit messages:
 - add functional dispatch feature

Changes: https://git.openjdk.org/jextract/pull/287/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=287&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903947
  Stats: 294 lines in 8 files changed: 263 ins; 6 del; 25 mod
  Patch: https://git.openjdk.org/jextract/pull/287.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/287/head:pull/287

PR: https://git.openjdk.org/jextract/pull/287