Add turn-key (and alloc-free) Sum{32,64,128} calls.

They all follow the `result := SumXXX(data []byte)' pattern, and are
exactly equivalent to:
`
    func Hash(data []byte) HXXX {
        hasher := NewXXX()
        hasher.Write(data)
        return hasher.SumXXX()
    }
'

In addition to being handy for most usages, it also greatly improves
small (few kilobytes and below) writes performance:

benchmark            old ns/op    new ns/op    delta
Benchmark32_1              118           26  -77.37%
Benchmark32_2              134           28  -78.96%
Benchmark32_4              116           26  -77.24%
Benchmark32_8              118           29  -75.42%
Benchmark32_16             121           32  -73.31%
Benchmark32_32             129           39  -69.69%
Benchmark32_64             145           54  -62.69%
Benchmark32_128            177           84  -52.37%
Benchmark32_256            246          150  -39.02%
Benchmark32_512            375          272  -27.47%
Benchmark32_1024           627          520  -17.07%
Benchmark32_2048          1105         1012   -8.42%
Benchmark32_4096          2091         1998   -4.45%
Benchmark32_8192          4062         3973   -2.19%

Benchmark128_1             122           33  -72.79%
Benchmark128_2             138           33  -75.87%
Benchmark128_4             159           35  -77.74%
Benchmark128_8             143           36  -74.41%
Benchmark128_16            122           35  -70.90%
Benchmark128_32            125           35  -71.76%
Benchmark128_64            133           44  -66.69%
Benchmark128_128           149           58  -60.94%
Benchmark128_256           181           85  -52.87%
Benchmark128_512           240          142  -40.83%
Benchmark128_1024          360          258  -28.33%
Benchmark128_2048          606          494  -18.48%
Benchmark128_4096         1064          955  -10.24%
Benchmark128_8192         1994         1884   -5.52%

benchmark             old MB/s     new MB/s  speedup
Benchmark32_1             8.46        37.47    4.43x
Benchmark32_2            14.88        70.95    4.77x
Benchmark32_4            34.21       151.56    4.43x
Benchmark32_8            67.65       275.42    4.07x
Benchmark32_16          131.31       495.74    3.78x
Benchmark32_32          246.21       818.67    3.33x
Benchmark32_64          440.35      1182.40    2.69x
Benchmark32_128         723.01      1518.96    2.10x
Benchmark32_256        1037.15      1700.58    1.64x
Benchmark32_512        1363.39      1877.52    1.38x
Benchmark32_1024       1631.87      1968.68    1.21x
Benchmark32_2048       1853.38      2022.71    1.09x
Benchmark32_4096       1958.09      2049.31    1.05x
Benchmark32_8192       2016.45      2061.41    1.02x

Benchmark128_1            8.14        30.15    3.70x
Benchmark128_2           14.48        59.99    4.14x
Benchmark128_4           25.01       112.88    4.51x
Benchmark128_8           55.92       218.30    3.90x
Benchmark128_16         131.09       450.86    3.44x
Benchmark128_32         254.00       905.84    3.57x
Benchmark128_64         479.22      1443.76    3.01x
Benchmark128_128        856.81      2201.02    2.57x
Benchmark128_256       1408.09      2999.88    2.13x
Benchmark128_512       2132.96      3592.97    1.68x
Benchmark128_1024      2843.22      3963.74    1.39x
Benchmark128_2048      3374.67      4144.65    1.23x
Benchmark128_4096      3849.06      4285.80    1.11x
Benchmark128_8192      4106.64      4347.12    1.06x