array sample slower than indexing into rand(size-1)

[cblake]

Also, for what it's worth, I think `proc rand*(r: var Rand; max Natural)` could 
be profitably upgraded/sped up with the techniques of [Lemire 
2018](https://r-libre.teluq.ca/1437/). But you can also just use 
`random.next()` and do that on your own. { It would also be unsurprising if 
@mratsim had that implemented deep in the corners of one of his large 
libraries... :-) }