fix(hax-lib/macros): handle correctly &mut Self arguments in ensures

[W95Psp]

This PR fixes #1266.

Consider the following Rust:

    #[hax_lib::attributes]
    trait T {
        #[hax_lib::ensures(|_|true)]
        fn v(x: &mut Self);
    }

The ensures clause was expanded (running just expand within hax repo, on the test crate) as:

    fn ensures<Self_>(x: &Self_, (x_future): (&Self)) -> bool
    where
        Self_:,
    {
        true
    }

The issue here is that we extract the various &mut arguments before renaming Self into Self_.
This PR fixes that, now the Rust expansion is:

    fn ensures<Self_>(x: &Self_, (x_future): (&Self_)) -> bool
    where
        Self_:,
    {
        true
    }

[karthikbhargavan]

Looks good to me.

[W95Psp]

GHA failed for macos-12 with a unknown error and a deprecated macos-12 error, I udpated the GHA to use macos-13