Coerce ints to longs during serialization

[mbossenbroek]

Clojure's value equality states that integers and longs of the same value are equal, but because pigpen serializes them differently, they are treated as not equal in joins.

=> (= (int 42) (long 42))
true

We should convert all ints to longs before serializing, with an option to disable this behavior. Also, look into how floats/doubles are handled.