Also decoupled the factorisation methods; now factor::factor contains
the logic that chains the different algorithms and aggregates results.
As a side-effect, rho::factor now performs extraneous allocations (as each
recursive step creates a new `Factors` value, which is then aggregated into
the previous one) but there is no significant performance impact.
It is clearer to see what is going on, as opposed to passing around an
unmarked `Vec<u64>`, and there is a single place to add invariants checks.
This is also a more compact memory representation: each prime factor is
represented only once, with an additional byte for multiplicity. The
performance impact is however not significant.
