Implement `slice_to_ascending_slice`

[flexatone]

def slice_to_ascending_slice(
        key: slice,
        size: int
        ) -> slice:
    '''
    Given a slice, return a slice that, with ascending integers, covers the same values.

    Args:
        size: the length of the container on this axis
    '''
    key_step = key.step
    key_start = key.start
    key_stop = key.stop

    if key_step is None or key_step > 0:
        return key

    # will get rid of all negative values greater than the size; but will replace None with an appropriate number for usage in range
    norm_key_start, norm_key_stop, norm_key_step = key.indices(size)

    # everything else should be descending, but we might have non-descending start, stop
    if key_start is not None and key_stop is not None:
        if norm_key_start <= norm_key_stop: # an ascending range
            return EMPTY_SLICE

    norm_range = range(norm_key_start, norm_key_stop, norm_key_step)

    # derive stop
    if key_start is None:
        stop = None
    else:
        stop = norm_range[0] + 1

    if key_step == -1:
        # gets last realized value, not last range value
        return slice(None if key_stop is None else norm_range[-1], stop, 1)

    return slice(norm_range[-1], stop, key_step * -1)