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.

r? @zwarich
f? @cgaebel 
Should this impl, and the one for `namespace::Namespace` be in this module? Ideally I'd like to break this out into its own cargo package one day, since a bloom filter is a relatively generic data structure.