Right shift operation semantics difference

[timsherwood]

Jose Renau at UCSC pointed out that PyRTL right shift semantics are different that the verilog right shift operations. It is unclear if we should follow verilog semantics, but we should be make such a decision purposefully. If we want to keep the semantics as they are now we should clearly document that it is different and double check that the verilog generation is correct.