Fix ensures for unit-returning functions with &mut inputs

[W95Psp]

Fixes #844, and #845

Fixes this kind of code:

#[hax_lib::ensures(|_| true)]
fn issue_844(_x: &mut u8) {}

// From issue #845
mod ensures_on_arity_zero_fns {
    #[hax_lib::requires(true)]
    #[hax_lib::ensures(|_x| true)]
    fn doing_nothing() {}
    #[hax_lib::requires(true)]
    #[hax_lib::ensures(|x| x > 100)]
    fn basically_a_constant() -> u8 {
        127
    }
}

Open this code snippet in the playground

[karthikbhargavan]

I tested it and it resolves my two issues. Thanks!

[W95Psp]

Thanks will put it in merge queue when the over is done