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/frc971/control_loops/drivetrain/libspline.cc b/frc971/control_loops/drivetrain/libspline.cc
index 6ea23c6..f275c17 100644
--- a/frc971/control_loops/drivetrain/libspline.cc
+++ b/frc971/control_loops/drivetrain/libspline.cc
@@ -121,8 +121,9 @@
 Trajectory *NewTrajectory(DistanceSpline *spline, double vmax,
                           int num_distance) {
   return new Trajectory(
-      spline, ::y2020::control_loops::drivetrain::GetDrivetrainConfig(), vmax,
-      num_distance);
+      DistanceSpline(*spline),
+      ::y2020::control_loops::drivetrain::GetDrivetrainConfig(), nullptr, -1,
+      vmax, num_distance);
 }
 
 void deleteTrajectory(Trajectory *t) { delete t; }