trapq: Add new trapq.c file with code for trapezoidal velocity handling

Move the "struct move" code from itersolve.c to new file trapq.c.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/chelper/kin_polar.c

@@ -9,6 +9,7 @@
 #include <string.h> // memset
 #include "compiler.h" // __visible
 #include "itersolve.h" // struct stepper_kinematics
+#include "trapq.h" // move_get_coord
 
 static double
 polar_stepper_radius_calc_position(struct stepper_kinematics *sk, struct move *m