Function normalize 

pub fn normalize(expr: &AlgExpr) -> AlgExpr

Produce a canonical form for expr:

• For commutative Add and Mul, sort the operand string representations so that a + b and b + a both become the same canonical form.
• Flatten nested Add(Add(a,b),c) → Add(a, Add(b,c)).
• Flatten nested Mul(Mul(a,b),c) → Mul(a, Mul(b,c)).