Speedup the ASCII decoder

Speedup the ASCII decoder

It is faster for long string and a little bit faster for short strings,
benchmark on Linux 32 bits, Intel Core i5 @ 3.33GHz:

./python -m timeit 'x=b"a"' 'x.decode("ascii")'
./python -m timeit 'x=b"x"*80' 'x.decode("ascii")'
./python -m timeit 'x=b"abc"*4096' 'x.decode("ascii")'

length |   before   | after
-------+------------+-----------
     1 | 0.234 usec | 0.229 usec
    80 | 0.381 usec | 0.357 usec
12,288 |  11.2 usec |  3.01 usec