differentiator: node, parser: remove custom ctor of Node::Power -- we have add_child() for that.
differentiator: node: implement recursive cloning of the tree.
expr: add fp_cmp() for correct comparison of floating-point values.
differentiator: visitor-simplify: add a visitor for simplifying an expression against one of its variables.
differentiator: visitor-simplify: implement folding (merging) of nested arithmetic nodes of same type.