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>