Add the value twopi to RIO.Prelude

[karamellpelle]

It is more natural to have a symbol for the value 2·π than π. Think about it: A circle is defined from a radius (the circle is all the points having distance radius) and so it it would be nice to have a symbol denoting the value of the circumference of a circle with unit radius (1). At least such a symbol would be more natural than having a symbol for half the circumference (π). Pi is wrong and should be removed from mathematics, that's my opinion.

Suggestion: Since RIO introduces a completely new Prelude, we have a good opportunity to introduce the value twopi :: Floating a => a. That's a value I've been missing.

PS. I just started programming with rio and I love it!

[snoyberg]

Fairly lukewarm on it. We don't typically attempt to add brand new things to rio without a strong motivation. A debate like pi-vs-tau doesn't seem to fit in there. That said, I don't have a lot of experience in doing this kind of work in Haskell and don't feel very strongly about this.