Use explicit flatbuffer IDs in y2017 and newer.

Non-explicit ids are risky.  We've seen backwards incompatible
changes...

Change-Id: Id6ceebe031ac80430191f367635d0e951c3d2cbc
diff --git a/y2020/control_loops/superstructure/superstructure_position.fbs b/y2020/control_loops/superstructure/superstructure_position.fbs
index ed4ba18..f139381 100644
--- a/y2020/control_loops/superstructure/superstructure_position.fbs
+++ b/y2020/control_loops/superstructure/superstructure_position.fbs
@@ -4,29 +4,29 @@
 
 table ShooterPosition {
   // Flywheel angle in radians, positive is shooting.
-  theta_finisher:double;
+  theta_finisher:double (id: 0);
 
   // Kicker angle in radians of the slowest (lowest) wheel, positive is
   // accelerating the ball toward the shooter.
-  theta_accelerator_left:double;
-  theta_accelerator_right:double;
+  theta_accelerator_left:double (id: 1);
+  theta_accelerator_right:double (id: 2);
 }
 
 table Position {
   // Zero is at the horizontal, positive towards the front (radians).
-  hood:frc971.AbsolutePosition;
+  hood:frc971.AbsolutePosition (id: 0);
 
   // Position of the intake. 0 when four-bar is vertical, positive extended.
-  intake_joint:frc971.AbsolutePosition;
+  intake_joint:frc971.AbsolutePosition (id: 1);
 
   // See goal for definition of 0
-  turret:frc971.PotAndAbsolutePosition;
+  turret:frc971.PotAndAbsolutePosition (id: 2);
 
   // Position of the kicker and flywheel
-  shooter:ShooterPosition;
+  shooter:ShooterPosition (id: 3);
 
    // Position of the control panel, relative to start, positive counterclockwise from above.
-  control_panel:frc971.RelativePosition;
+  control_panel:frc971.RelativePosition (id: 4);
 }
 
 root_type Position;