Reporting latency

[robbert-vdh]

There are still functions to set port latency ranges (and wow is that JACK API ever overly complicated), but those need to be called from the latency callback and that has been completely removed in #148. I can add it back as another Option<LatencyHandler> argument to client.activate_async(). It may also be useful to have the LatencyHandler expose methods to set the latency for a port or entire client to a single value and have it do all of the ceremonial computations for you. And also move the latency setting functions from Port to LatencyHandler while I'm at it since it can only be misused now.

Thoughts?