tr_unique doesn't work in some cases

[emricksinisonos]

The tr_unique function doesn't remove Tr duplicates if they are not consecutive within the list of trs of a state. The same behaviour happens in OpenFST so adding a new test case for this situation will break comparison tests.
The following example can reproduce the issue:

    #[test]
    fn test_tr_map_unique_1() -> Result<()> {
        let mut fst_in = VectorFst::<ProbabilityWeight>::new();

        let s1 = fst_in.add_state();
        let s2 = fst_in.add_state();

        fst_in.add_tr(s1, Tr::new(1, 2, ProbabilityWeight::new(1.0), s2))?;
        fst_in.add_tr(s1, Tr::new(1, 2, ProbabilityWeight::new(2.0), s2))?;
        fst_in.add_tr(s1, Tr::new(1, 2, ProbabilityWeight::new(1.0), s2))?;

        fst_in.set_start(s1)?;
        fst_in.set_final(s2, ProbabilityWeight::one())?;

        let mut fst_out = VectorFst::<ProbabilityWeight>::new();

        let s1 = fst_out.add_state();
        let s2 = fst_out.add_state();

        fst_out.add_tr(s1, Tr::new(1, 2, ProbabilityWeight::new(1.0), s2))?;
        fst_out.add_tr(s1, Tr::new(1, 2, ProbabilityWeight::new(2.0), s2))?;

        fst_out.set_start(s1)?;
        fst_out.set_final(s2, ProbabilityWeight::one())?;

        tr_unique(&mut fst_in);

        assert_eq!(fst_in, fst_out);

        Ok(())
    }

It can easily be solved by adding a check on the weight value of a transition in tr_compare :

pub(crate) fn tr_compare<W: Semiring>(tr_1: &Tr<W>, tr_2: &Tr<W>) -> Ordering {
    if tr_1.ilabel < tr_2.ilabel {
        return Ordering::Less;
    }
    if tr_1.ilabel > tr_2.ilabel {
        return Ordering::Greater;
    }
    if tr_1.olabel < tr_2.olabel {
        return Ordering::Less;
    }
    if tr_1.olabel > tr_2.olabel {
        return Ordering::Greater;
    }
    if tr_1.weight < tr_2.weight {
        return Ordering::Less;
    }
    if tr_1.weight > tr_2.weight {
        return Ordering::Greater;
    }
    if tr_1.nextstate < tr_2.nextstate {
        return Ordering::Less;
    }
    if tr_1.nextstate > tr_2.nextstate {
        return Ordering::Greater;
    }
    Ordering::Equal
}