1st draft of complete class-based std.random successor

[bearophile]

Joseph Rushton Wakeling:

> Latest patches just pushed to repo make the randomSample => 
> sample change and introduce a fast uniform01 and 
> uniform01Distribution :-)

They seem good.

More ideas:

"Three suggestions for std.random":
https://d.puremagic.com/issues/show_bug.cgi?id=4851

"Strongly pure random generator":
https://d.puremagic.com/issues/show_bug.cgi?id=5249

I hope a gaussian (normal distribution) generator is planned or 
present.

Bye,
bearophile