Simplify the expression tree

[lovasoa]

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>.

[jcavat]

is it still relevant after last commits ?

[lovasoa]

removing Subexpr would significantly simplify the code.
And restructuring the struct to have only a sum of multiplications would improve performance