Refactor trajectory generation to separate process

This pulls all the trajectory planning into a TrajectoryGenerator class,
which produces a Trajectory spline that the drivetrain code can consume
and use to track the spline.

Broadly speaking, this change:
- Separates the Trajectory class into a generation class and a
  FinishedTrajectory class, where the generator produces a flatbuffer
  and the FinishedTrajectory reads all the required information from
  the flatbuffer.
- Add an option for serialization/deserialization of a DistanceSpline.
- Removes some dead code from Trajectory class (mostly having to do with
  the old feedback algorithm).
- Uses floats in more places, to keep the size of the Trajectory
  flatbuffer under control
- Update the tests & autonomous code to use the new spline code.

Further work that may make sense:
- Experiment with alternatives to current structure of the Trajectory
  flatbuffer to see whether (a) the size is a problem; and (b) if so,
  what we should do about it.
- Add shims to allow replaying logfiles with old-style spline goals.

Change-Id: Ic80ce4e384ec4d1bd22940580e3652ecd305b352
diff --git a/y2016/BUILD b/y2016/BUILD
index 8c695c8..ae12689 100644
--- a/y2016/BUILD
+++ b/y2016/BUILD
@@ -40,6 +40,7 @@
         "//frc971/autonomous:auto_fbs",
         "//frc971/control_loops/drivetrain:drivetrain_goal_fbs",
         "//frc971/control_loops/drivetrain:drivetrain_status_fbs",
+        "//frc971/control_loops/drivetrain:spline_goal_fbs",
         "//frc971/queues:gyro_fbs",
         "//y2016/actors:autonomous_action_lib",
         "//y2016/actors:superstructure_action_lib",