use std::fill

For a cwx benchmark (100^3 supervoxel segmentation), this reduces
the runtime before/after:

./cwx seg-100.h5 seg  104,92s user 0,36s system 99% cpu
./cwx seg-100.h5 seg  15,10s user 0,06s system 99% cpu
Merge pull request #8 from thorbenk/speedups

use std::fill