Normal/Gaussian random number generation for D

[jerro]

> isNormalRandomNumberEngine, and I don't know if you want to add 
> some extra type checks to some of your Ziggurat-related 
> functions).

Yes, I need to add some checks. Currently, there aren't any :D

> Lastly, we need to make sure that this new functionality is 
> well documented. Perhaps a good start here could be for people 
> to take a look at the code and describe what isn't clear or 
> easy to understand just from looking ... ?

By functionality, do you mean just the API or also the 
implementation? I assume my Ziggurat algorithm implementation 
isn't easy to understand without some kind of description. I'll 
need to write a ton of comments in there.