Return run-length encoding string from `img`.

def rle_encode(img:NPArrayMask)->str:
    "Return run-length encoding string from `img`."
    pixels = np.concatenate([[0], img.flatten() , [0]])
    runs = np.where(pixels[1:] != pixels[:-1])[0] + 1
    runs[1::2] -= runs[::2]
    return ' '.join(str(x) for x in runs)