* Introduce Pacer interface and ConstantPacer.

The Pacer interface allows vegeta's attack rate to be customized.
Attack code keeps track of the elapsed attack duration and the number
of hits sent and provides these to Pace, which must return a Duration
for the attack code to sleep for before sending the next hit. Pace may
also return true for the second stop value to terminate the attack.

This interface allows vegeta to model a wider range of attack scenarios,
such as diurnal request patterns or load-test behaviour, where the
attack rate increases until the target begins to serve errors.

* Add SinePacer, for sinusoidal attack rates.

SinePacer is a Pacer that describes attack request rates with the
equation Rate = Mean * Amplitude * sin(Offset + t * (2𝛑 / Period)).

Both the Mean and Amplitude rates are expressed with ConstantPacer.