Implement operations for rug::Complex

[IntrepidPig]

• add
• sub
• mul
• div
• pow
• sqrt
• nrt
• abs
• sin
• cos
• tan
• asin
• acos
• atan
• atan2
• floor
• ceil
• round
• log

[autumnull]

I don't think atan2 makes sense with Complex numbers.

ceil and round can be implemented in the same way as floor.

nrtis a little more complex - if this returns a Multiple under the same rules as sqrt it could be quite slow to compute all the roots for higher values of n, but I think using rug's root_of_unity function you could find the primary root using pow(1/n) and then multiply it by the roots of unity to get each separate result... This would only work for integer values of n though.