feature: reintroduce ID-based based equality/hashing to #[salsa::Supertype]

[davidbarsky]

After #677 lands, we should re-add the following code to #[salsa:Supertype]:

let std_traits = quote! {
    impl #impl_generics ::core::cmp::PartialEq for #enum_name #type_generics
    #where_clause {
        #[inline]
        fn eq(&self, __other: &Self) -> bool {
            zalsa::AsId::as_id(self) == zalsa::AsId::as_id(__other)
        }
    }

    impl #impl_generics ::core::hash::Hash for #enum_name #type_generics
    #where_clause {
        #[inline]
        fn hash<__H: ::core::hash::Hasher>(&self, __state: &mut __H) {
            ::core::hash::Hash::hash(&zalsa::AsId::as_id(self), __state);
        }
    }
};

It was removed in this commit because ID-based hashing and equality conflicted with with rust-analyzer's interner, which outlives the Salsa database in tests. Once rust-analyzer gets rid of its native interner in favor of Salsa's interner, then I think it'd safe to re-add this functionality. Alternatively, we can reimplement #[salsa::Supertype] as an attribute macro (instead of a derive) and add options to disable deriving Hash/Equality, but I'm too lazy to do that right now.

[Veykril]

Imo we should not emit these impls by default, we could add a derive helper #[supertype(eq, hash)] to emit them but otherwise the user may want to have variant tag based identity still.