add bloom_filter module

Reviewer: @kenchiang

See http://pages.cs.wisc.edu/~cao/papers/summary-cache/node8.html for more info on Bloom filters.

I plan to use this in the IVS tcam module, as well as to replace an existing open-coded Bloom filter in the IVS upcall processing code. Using it in the tcam module gives me a speedup of 36%.