Factor implementation and change interface.

When constructing a Bloom filter, one now has to pass a HashPolicy instance to
it. This separates more clearly the concerns of hashing and Bloom filter
management.

This commit also changes the interface to initialize Bloom filters: there exist
now two initialization functions, one for each type:

  (1) bloomfilter_basic_init(fp: double,
                             capacity: count,
                             name: string &default=""): opaque of bloomfilter

  (2) bloomfilter_counting_init(k: count,
                                cells: count,
                                max: count,
                                name: string &default=""): opaque of bloomfilter

The BiFs for adding elements and performing lookups remain the same. This
essentially gives us "BiF polymorphism" at script land, where the
initialization BiF constructs the most derived type while subsequent BiFs
adhere to the same interface.

The reason why we split up the constructor in this case is that we have not yet
derived the math that computes the optimal number of hash functions for
counting Bloom filters---users have to explicitly parameterize them for now.