rb_shape_transition_shape_capa: use optimal sizes transitions

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
```

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