Arithmetic operations on references (e.g. `Add`, `Mul`) are incorrectly flagged as impure

[chisa22]

Prusti reports an "impure function" error when using standard arithmetic operators (like + or *) on references inside specifications.
Reproduction

use prusti_contracts::*;

#[pure]
#[requires(a + b <= std::u32::MAX)] // Error: use of impure function "std::ops::Add::add"
fn sum_ref(a: u32, b: &u32) -> u32 {
    a + *b
}

#[pure]
#[requires(a * b > 0)] // Error: use of impure function "std::ops::Mul::mul"
fn mul_refs(a: &i32, b: &i32) -> i32 {
    a * b
}

fn main() {}

Actual Behavior

error: [Prusti: invalid specification] use of impure function "std::ops::Add::add" in pure code is not allowed
error: [Prusti: invalid specification] use of impure function "std::ops::Mul::mul" in pure code is not allowed

Expected Behavior
Arithmetic operations on primitive references should be treated as pure, just like operations on values.