Add rounding operations to Data.Rational

[gallais]

This interesting article suggests useful operations we may want to have on rational numbers:

round floor ceiling truncate : ℚ → ℤ × ℚ

together with the proofs of their properties, of course.

[MatthewDaggitt]

Hmm I'd be tempted to have the standard signature ℚ → ℤ for those operations then have another operator frac : ℚ →ℚ that returns the fractional part. Presumably they're simply going to be implemented by the _/_ and _%_ operators respectively so it's not like they have any computational overlap.

[barrettj12]

I've just opened a PR #1585 which implements the suggested functions. I have not been able to prove any properties about said functions. Should we open a new issue for the implementation of those properties?