Cycle detection

[WhittlesJr]

One of the stickiest issues I deal with when debugging problems with fun-map is that it will silently perform infinite loops if you have a dependency cycle in your fws. It would really help improve the robustness of the library if it could detect an infinite cycle and point out which keys are to blame.

[robertluo]

Good point, it can be done by tracking the invocation order in runtime, like using a trace-fn, however, I am not very sure if we should leave it to users.

[WhittlesJr]

weavejester's dependency library has cycle detection. I wonder if fun-map could use it directly, or if that's too much refactoring, at least use it as inspiration for its own implementation.

[robertluo]

Great suggestion.