Added JumpStepWrapper

[andyElking]

Hi Patrick,

I factored the jump_ts and step_ts out of the PIDController into JumpStepWrapper (I'm not very set on this name, lmk if you have ideas). I also made it behave as we discussed in #483. In particular, the following three rules are maintained:

1. We always have t1-t0 <= prev_dt (this is checked via eqx.error_if), with inequality only if the step was clipped or if we hit the end of the integration interval (we do not explicitly check for that).
2. If the step was accepted, then next_dt must be >=prev_dt.
3. If the step was rejected, then next_dt must be < t1-t0.

We achieve this in a very simple way here: https://github.com/patrick-kidger/diffrax/blob/78b122adf39b2f8d26a79d0ac239a2fb675653a1/diffrax/_step_size_controller/jump_step_wrapper.py#L119-L123

The next step is to add a parameter JumpStepWrapper.revisit_rejected_steps which does what you expect. That will appear in a future commit in this same PR.