[libre-riscv-dev] div/mod algorithm written in python

[Jacob Lifshay]

I refactored core.py into core.py and div_pipe.py.
I also added tests that demonstrate integer and fractional division:
test_algorithm.py:test_int_div/test_fract_div

I added most of the test code for DivPipeCore, but there's a bug
somewhere (probably in core.py) that makes nmigen generate invalid
rtlil (yosys complains when trying to load it).

Feel free to help out with bug-hunting, if you like.

Jacob