Added OperationParser and the corresponding ConstatantCreator.
OperationParser will parse math-expressions, like GenSolver, but will create a Operation of nested Operations instead.
So e.g. "1+x" could yield: Adder{1, x} that will return 1+args when solved.
Minor changes to GenSolver to make this work. Might have to split GenSolver into AbstractGenSolver and GenSolver in the future.