Implement operations for rug::Rational

[IntrepidPig]

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

[autumnull]

I think the majority of these unimplemented ones won't be achievable within rug::Rational... I'm fairly sure that the only ones that make sense are

• add/sub/mul/div
• abs
• floor
• ceil
• round

because all the others can result in irrational numbers (unless you restricted "pow" to only work for non-negative integers).