Optimise bucket placement.

Use a "generation vector" instead of a hashmap for storing the elements
of the bucket currently being placed. This gives a 2x speed-up
e.g. creating a phf_set out of my 99171-line /usr/share/dict/words took
1 second previously and now takes 0.4-0.5.
Merge pull request #11 from huonw/speed++

Optimise bucket placement.