Add convenience functions to transform between edgedb.Duration and time.Duration

[lsdch]

This is meant to improve usability of Duration values and better integrate with the Go ecosystem. This also makes the unit (micro/nanoseconds) difference between Duration and time.Duration more obvious, hopefully reducing the likelihood of introducing hard to detect bugs such as:

func ConvertDuration(d edgedb.Duration) time.Duration {
  return time.Duration(int64(d)) 
  // change from microseconds to nanoseconds representation is not taken care of, 
  // and the compiler can not help to prevent this
}

[fmoor]

Any last changes before I merge?

[lsdch]

Nothing comes to mind, looks like it is ready :rocket:

[fmoor]

released in v0.17.0