Preliminary submission - std.rational and std.typelist

[bearophile]

FG:

> Not sure though whether what's possible in Mono's GC can be 
> ported to D.

With "developed in conjunction" I meant written with then, not 
just ported (that means copied and then adapted). Go, C#, Rust 
and D need GCs with different characteristics, but not totally 
different characteristics.

Compared to D, I think in C# pinned objects and low level coding 
is less common.

Bye,
bearophile