cksum: add massive performance improvements

I did this in response to [this post](http://www.reddit.com/r/programming/comments/27yccu/crossplatform_rust_rewrite_of_the_gnu_coreutils/ci5vehd) on Reddit.  On my machine, the uutils version tends to run faster than the GNU version, but still lags behind the Mac version (the Mac version must be _very_ highly optimized).  Benchmarks were done following the same setup as in the Reddit post:
```bash
$ FILES=$(find . -name '*.rs')
$ FILES="$FILES $FILES $FILES $FILES"
$ FILES="$FILES $FILES $FILES $FILES"
```

#### Benchmarks
uutils:
```bash
bash-4.3$ time for a in {1..10}; do build/cksum $FILES; done > /dev/null

real	0m0.639s
user	0m0.464s
sys	0m0.170s
```

GNU:
```bash
bash-4.3$ time for a in {1..10}; do gcksum $FILES; done > /dev/null

real	0m0.651s
user	0m0.463s
sys	0m0.188s
```

Mac:
```bash
bash-4.3$ time for a in {1..10}; do cksum $FILES; done > /dev/null

real	0m0.544s
user	0m0.412s
sys	0m0.140s
```
This may be faster as `bytes.slice_to(num_bytes).iter()`, but you would have to measure it.