stepcompress: Do all step rounding in C code

Commits f0cefebf and 8f331f08 changed the way the code determined what
steps to take on fractional steps.  Unfortunately, it was possible in
some situations for the C code to round differently from the python
code which could result in warnings and lost steps.

Change the code so that all fractional step handling is done in the C
code.  Implementing the step rounding logic in one location avoids any
conflicts.

In order to efficiently handle the step rounding in the C code, the C
code has also been extended to directly send the "set_next_step_dir"
command.

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

klippy/stepper.py

@@ -69,9 +69,9 @@ class PrinterStepper:
             mcu_time = self.mcu_enable.print_to_mcu_time(move_time)
             self.mcu_enable.set_digital(mcu_time, enable)
         self.need_motor_enable = True
-    def prep_move(self, move_time, sdir):
+    def prep_move(self, move_time):
         mcu_time = self.mcu_stepper.print_to_mcu_time(move_time)
-        self.mcu_stepper.set_next_step_dir(mcu_time, sdir)
+        self.mcu_stepper.check_reset(mcu_time)
         if self.need_motor_enable:
             self.motor_enable(move_time, 1)
             self.need_motor_enable = False