Implement basic ops for PolyExpr type

[DoHoonKim8]

In a future PR you could consider implementing some operands for this type like this:

halo2-lasso/plonkish_backend/src/util/expression.rs

Lines 517 to 520 in 15f1f1a

	impl_expression_ops!(Mul, mul, Product, Expression<F>, std::convert::identity);
	impl_expression_ops!(Mul, mul, Scaled, F, std::convert::identity);
	impl_expression_ops!(Add, add, Sum, Expression<F>, std::convert::identity);
	impl_expression_ops!(Sub, sub, Sum, Expression<F>, Neg::neg);

This allows doing things like this:

let x = Var(i);
let y = Var(i + 8);
let sum = x + y;

Originally posted by @ed255 in #4 (comment)