Separate the Instrument from the Snap

- before, the Instrument object would contain the snap
  - this seemed like a good idea at the time
  - but isn't, because we cannot instantiate an Instrument
- introduces the Telemetry::Instrument::Snap role
  - which is mostly what Telemetry::Instrument was
  - minus "formats" and "columns"
- should make it easy to make a Telemetry::Instrument::AdHoc
  - which could e.g. just take a number of variables to snap