Add a test for creating a callback and passing it to qsort

[certik]

Here is a rough idea how it can work:

Comparator = Callable[[Any, Any], int]
@ccall
def qsort(arr: Sequence[Any],
          num_elems: int,
          elem_size: int,
          comp: Comparator) -> None:
    pass

The types have to be adjusted. And we want to use it like:

def sort_f16_idx_val_ascending(iv_ptr: CPtr, num_elements: i32) -> None:
    iv = ctypes.cast(iv_ptr, ctypes.POINTER(gd_lab_4_idx_val))
    qsort(iv, num_elements, sizeof(gd_lab_4_idx_val), f16_idx_val_cmp_ascending)

where the f16_idx_val_cmp_ascending function is the comparator. And we want this to call the qsort function in C.

I think we have most of these pieces in LPython/LFortran, we just need to write a test for this and ensure everything works and fix any bugs.

[certik]

To test this, I would create our own C version of "qsort" with a callback, that way we can control the types exactly and make it work.