Unsoundness: Tracked-fields with no_eq can store references

[MichaReiser]

#[salsa::tracked]
pub(crate) struct Expression<'db> {

    #[no_eq]
    pub(crate) test: &'db str,

}

This compiles when it should not. We still need to enforce the Update trait for test or, have a dedicated trait (because implementing Update is a bit non-sensical)