`time_change` should integrate cumulative inter-arrival segments

[rsenne]

time_change(h, pp::InhomogeneousPoissonProcess) currently computes the compensator at each event time and at tmax via independent quadrature calls. Independent calls have independent ulp-level rounding errors, so the transformed times can come out of order or slightly past new_tmax. Switching to a cumulative integration over inter-arrival segments removes both failure modes by construction with no change in cost.