factor::Decomposition: Optimise as a factor is never added twice
factor::Decomposition: Optimise as a factor is never added twice

The invariant is checked by a debug_assert!, and follows from the previous
commit, as `dec` and `factors` only ever contains coprime numbers:
  - true at the start: factor = ∅ and dec = { n¹ } ;
  - on every loop iteration, we pull out an element `f` from `dec` and either:
    - discover it is prime, and add it to `factors` ;
    - split it into a number of coprime factors, that get reinserted into `dec`;
      the invariant is maintained, as all divisors of `f` are coprime with all
      numbers in `dec` and `factors` (as `f` itself is coprime with them.

As we only add elements to `Decomposition` objects that are coprime with the
existing ones, they are distinct.

uutils/coreutils,uutils/coreutils,uutils/coreutils