[WIP] Subs class

At the moment the dummy variable for derivative is `"_x"`. 
Needs better methods for, `Derivative::diff` and `Subs::subs`
Your code is not wrong, but you can also just use `v[i] = symbol("_x");`, which does the same. I think it's generally better to use such functions like `symbol` rather than using `rcp` directly. Less prone to errors.