We can pull the string on our current 'all-static' approach for jextract further by replacing the `ofAddress` factory in the functional interface classes we generate with a static invoke method.

This would avoid clients having to create an intermediate instance of the FI in order to invoke a function pointer they got back from native code.

    MemorySegment ptr = ...
    FP.ofAddress(ptr, someArena).apply(42);

->

    MemorySegment ptr = ...
    FP.invoke(ptr, 42);

It would also make it easier to deal with function pointers which return by-value structs, since the proposed 'invoke' method could just accept an additional SegmentAllocator argument.