Impl Step for Phase

[niclashoyer]

I'm currently implementing PolyBLEP oscillators using the Signal trait. One problem that occurred is that I need a current step and phase at the same time. But as step is private in Phase it is not possible so get the current step.

What is the most ergonomic way to implement this? In this PR I just implemented Step for Phase, so that one can just call step on any Phase to get the internal step value.