Extend the Bloom `Filter` to support operands.

[lucanicoladebiasi]

As SDK user I wish to combine different Bloom filters to check if an element (address) is possibly part of multiple Thor's blocks get in different moments.

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Bloom Filter class depends from the parameters

• k : number of hash functions used to generate the filter,
• m : number of bits per key in the filter used to represent an entry in the set of elements the filter represents.

The two parameters are related: if we set n as the number of elements of the set generating the filter, k = ln(2) * m / n.

Different Bloom filters can't be combined if m and k were different when the filters were computed.

The goal of the ticket is

• to add to the Filter class the properties k and m;
• to use n as parameter to compute the best k by default once m is set. (m should depend on the probability of the of false positive accepted, hence is a function of n as well);
• to provide a method to check if different filters are compatible;
• to provide a method to merge filters in a new.