Refactor `synth`

[Tilman Hinnerichs]

• introduce eval(...) to take care of the different interpretation outputs of HerbInterpret.interpret(...)
• introduce stop_criterium(...) defining when to stop the search

[Reuben Gardos Reid]

@THinnerichs can we close now that #70 is published?

[Tilman Hinnerichs]

1. evaluate(...) should be able to take different kinds of problem specification. This is definitely for later.
2. stop_criterium is still open.

We have

 score = evaluate(problem, expr, symboltable, shortcircuit=shortcircuit, allow_evaluation_errors=allow_evaluation_errors)
if score == 1
    return (candidate_program, optimal_program)
elseif score >= best_score
    best_score = score
    best_program = candidate_program
end

but we may want to stop using other criteria than score == 1.