Use ticket #41: use array for sequence decode where possible

PyPy, before:
        10000 loops, best of 6: 54.9 usec per loop

PyPy, after:
        100000 loops, best of 6: 7.17 usec per loop

CPython, before:
        100 loops, best of 6: 2.46 msec per loop

CPython, after:
        100000 loops, best of 6: 6.42 usec per loop