differentiator: node, visitor-simplify: implement add_child_front() method and use it to insert the constant to mul-div nodes.
differentiator: visitor-simplify: push the add-sub sign into mul-div node's constant where appropriate.

If mul-div is the only child of an add-sub (and it has a negative sign), then
we still can get rid of the add-sub node entirely by pushing the negative sign
into the mul-div node itself.