Simplify the expression tree #67

What would you think about simplifying the expression tree by

• Removing SubExpr (and replacing it with Add(Mul(-1, expr))
• Replacing AddExpr(left, right) by AddExpr(Vec<expr>), to avoid creating deep trees (that cause Simplify overflows the stack #37 )

This is not perfect, but should be easy to implement as a workaround for now. Would you accept such a PR ?

A more ambitious refactor that we can keep for later is to implement the expression as a simple HashMap<Variable, Coefficient>.