self in requires clause gets renamed to self__

[karthikbhargavan]

When adding pre- and post-conditions to an impl block, I found a situation where references to self in the annotations result in malformed F*.

struct S(pub  u8);

#[hax_lib::attributes]
impl S {
    #[hax_lib::requires(self.0 == 0)]
    fn f(&self) {}
}

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