Implement operations for rug::Complex

[IntrepidPig]

• [x] add
• [x] sub
• [x] mul
• [x] div
• [x] pow
• [x] sqrt
• [ ] nrt
• [x] abs
• [x] sin
• [x] cos
• [x] tan
• [x] asin
• [x] acos
• [x] atan
• [ ] atan2
• [x] floor
• [ ] ceil
• [ ] round
• [x] 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.