Implement broken/missing arithmetic methods

[mark-koch]

• [ ] __pow__ for int and float. Either add native support in Hugr or implement via loop? Note that there are also some typing issues: 2 ** 2 = 2 is int but 2 ** -1 = 0.5 is float in Python!
• [ ] __round__ for float
• [ ] __lshift__ etc are broken: Shifts in the arithmetic extension treat the RHS as unsigned. We either have to change this or have special logic that treats compilation of negative shifts
• [ ] Double check the Hugr spec for iabs: Does it return signed or unsigned ints?
• [x] Use devision ops with panic once they are added to Hugr (c.f. #52)