When generic instance variable has a shape, it is marked movable. If it
it transitions to too complex, it needs to update references otherwise
it may have incorrect references.

Previously the growth was 3(embed), 6, 12, 24, ...

With this change it's now 3(embed), 8, 16, 32, 64, ... by default.

However, since power of two isn't the best size for all allocators,
if `malloc_usable_size` is vailable, we use it to discover the best
offset.

On Linux/glibc 2.35 for instance, the growth will be 3(embed), 7, 15, 31
to avoid wasting 8B per object.

Test program:

```c

size_t test(size_t slots) {
    size_t allocated = slots * VALUE_SIZE;
    void *test_ptr = malloc(allocated);
    size_t wasted = malloc_usable_size(test_ptr) - allocated;
    free(test_ptr);
    fprintf(stderr, "slots = %lu, wasted_bytes = %lu\n", slots, wasted);
    return wasted;
}

int main(int argc, char *argv[]) {
    size_t best_padding = 0;
    size_t padding = 0;
    for (padding = 0; padding <= 2; padding++) {
        size_t wasted = test(8 - padding);
        if (wasted == 0) {
            best_padding = padding;
            break;
        }
    }

    size_t index = 0;
    fprintf(stderr, "=============== naive ================\n");

    size_t list_size = 4;
    for (index = 0; index < 10; index++) {
        test(list_size);
        list_size *= 2;
    }

    fprintf(stderr, "=============== auto-padded (-%lu) ================\n", best_padding);

    list_size = 4;
    for (index = 0; index < 10; index ++) {
        test(list_size - best_padding);
        list_size *= 2;
    }

    fprintf(stderr, "\n\n");
    return 0;
}
```

```
===== glibc ======
slots = 8, wasted_bytes = 8
slots = 7, wasted_bytes = 0
=============== naive ================
slots = 4, wasted_bytes = 8
slots = 8, wasted_bytes = 8
slots = 16, wasted_bytes = 8
slots = 32, wasted_bytes = 8
slots = 64, wasted_bytes = 8
slots = 128, wasted_bytes = 8
slots = 256, wasted_bytes = 8
slots = 512, wasted_bytes = 8
slots = 1024, wasted_bytes = 8
slots = 2048, wasted_bytes = 8
=============== auto-padded (-1) ================
slots = 3, wasted_bytes = 0
slots = 7, wasted_bytes = 0
slots = 15, wasted_bytes = 0
slots = 31, wasted_bytes = 0
slots = 63, wasted_bytes = 0
slots = 127, wasted_bytes = 0
slots = 255, wasted_bytes = 0
slots = 511, wasted_bytes = 0
slots = 1023, wasted_bytes = 0
slots = 2047, wasted_bytes = 0
```

```
==========  jemalloc =======
slots = 8, wasted_bytes = 0
=============== naive ================
slots = 4, wasted_bytes = 0
slots = 8, wasted_bytes = 0
slots = 16, wasted_bytes = 0
slots = 32, wasted_bytes = 0
slots = 64, wasted_bytes = 0
slots = 128, wasted_bytes = 0
slots = 256, wasted_bytes = 0
slots = 512, wasted_bytes = 0
slots = 1024, wasted_bytes = 0
slots = 2048, wasted_bytes = 0
=============== auto-padded (-0) ================
slots = 4, wasted_bytes = 0
slots = 8, wasted_bytes = 0
slots = 16, wasted_bytes = 0
slots = 32, wasted_bytes = 0
slots = 64, wasted_bytes = 0
slots = 128, wasted_bytes = 0
slots = 256, wasted_bytes = 0
slots = 512, wasted_bytes = 0
slots = 1024, wasted_bytes = 0
slots = 2048, wasted_bytes = 0
```

WeakMap can crash during compaction because the st_insert could allocate
memory.

If we're during incremental marking, then Ruby code can execute that
deallocates certain memory buffers that have been called with
rb_gc_mark_weak, which can cause use-after-free bugs.

This is an internal only function not exposed to the C extension API.
It's only use so far is from rb_vm_mark, where it's used to mark the
values in the vm->trap_list.cmd array.

There shouldn't be any reason why these cannot move.

This commit allows them to move by updating their references during the
reference updating step of compaction.

To do this we've introduced another internal function
rb_gc_update_values as a partner to rb_gc_mark_values.

This allows us to refactor rb_gc_mark_values to not pin

[Feature #19783]

This commit adds support for weak references in the GC through the
function `rb_gc_mark_weak`. Unlike strong references, weak references
does not mark the object, but rather lets the GC know that an object
refers to another one. If the child object is freed, the pointer from
the parent object is overwritten with `Qundef`.

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>

[Feature #18885]

For now, the optimizations performed are:

  - Run a major GC
  - Compact the heap
  - Promote all surviving objects to oldgen

Other optimizations may follow.

Introduce Universal Parser mode for the parser.
This commit includes these changes:

* Introduce `UNIVERSAL_PARSER` macro. All of CRuby related functions
  are passed via `struct rb_parser_config_struct` when this macro is enabled.
* Add CI task with 'cppflags=-DUNIVERSAL_PARSER' for ubuntu.

Classes are now exactly 80 bytes when embedded, which perfectly fits the
3rd size pool on 32 bit systems.

This commit will allow 32 bit systems to take advantage of VWA.