Use Gecko's simpler Bloom filter instead of one based on hash
stretching.

This preserves the usage of the Bloom filter throughout style recalc,
but the implementation is rewritten. Provides a 15% improvement on
Guardians of the Galaxy.
auto merge of #3394 : pcwalton/servo/rewrite-bloom-filter, r=zwarich

This preserves the usage of the Bloom filter throughout style recalc,
but the implementation is rewritten. Provides a 15% improvement on
Guardians of the Galaxy.

r? @zwarich
f? @cgaebel