Move to interface based hierarchy

[timtadh]

Right now fuzzbuzz/models has a traditional OO object hierarchy. Frankly, it is a mess. Many object hierarchies are actually sharing common methods (flow, satisfiable, value, etc...). These hierarchies are not really related but they are implicitly related.

I want to unify this situation by moving to interface definitions (except perhaps for the Value hierarchy which is actually using things like has_value from Value).