Improve the `Expr` string format

[Ted-Jiang]

We need improve the Expr fmt: like

let expr1 = and(a_1.clone(), b_2.clone())
            .and(a_2.clone())
            .and(b_1.clone());

        let expr2 = and(and(a_1.clone(), b_2.clone()), and(a_2.clone(), b_1.clone()));
        assert_eq!(expr1, expr2);

This is not equal but they have the same string format:

a = Int64(1) AND b = Int64(2) AND a = Int64(2) AND b = Int64(1)

Originally posted by @Ted-Jiang in #3859 (comment)

[andygrove]

This seems like a duplicate of #3865?

[Ted-Jiang]

This seems like a duplicate of #3865?

Yes, it bother me when writing test 😂

[Ted-Jiang]

@andygrove We must print the priority of the Expr.