Implement fmt::Display for all expressions

[ywqzzy]

impl fmt::Display for Operator {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let display = match &self {
            Operator::Eq => "=",
            Operator::NotEq => "!=",
            Operator::Lt => "<",
            Operator::LtEq => "<=",
            Operator::Gt => ">",
            Operator::GtEq => ">=",
            Operator::Plus => "+",
            Operator::Minus => "-",
            Operator::Multiply => "*",
            Operator::Divide => "/",
            Operator::Modulo => "%",
            Operator::And => "AND",
            Operator::Or => "OR",
            Operator::Like => "LIKE",
            Operator::NotLike => "NOT LIKE",
            Operator::RegexMatch => "~",
            Operator::RegexIMatch => "~*",
            Operator::RegexNotMatch => "!~",
            Operator::RegexNotIMatch => "!~*",
            Operator::IsDistinctFrom => "IS DISTINCT FROM",
            Operator::IsNotDistinctFrom => "IS NOT DISTINCT FROM",
            Operator::BitwiseAnd => "&",
            Operator::BitwiseOr => "|",
        };
        write!(f, "{}", display)
    }
}

Ref to the code in datafusion (datafusion/expr/src/operator.rs)