Preliminary submission - std.rational and std.typelist

[bearophile]

Arlen:

> I ran some benchmarks, and I don't see how the rosetta version 
> is more optimized:

My message says "See this implementation (that doesn't use an 
optimized GCD):". I have never said that code is more optimized.

What I have said is that your version probably should simplify 
its arguments inside the constructor, possibly with an optimized 
GCD.

Bye,
bearophile