[FEATURE] Reverse iteration for `Factorization`

Given a factorization from `factor(N)` (that's eager and already sorted in `f.pe`), it should be relatively easy to implement `iterate` for `Iterators.Reverse{T}`. Alternatively (preferably) declare the `pe` field public or provide an API to get read-only access to `pe` as a vector without allocating. (hope I haven't missed the way to do this if it's already possible)