Add ability to sample elements from openArray according to a weight array (#10072)

* Add the ability to sample elements from an openArray according to a parallel
array of weights/unnormalized probabilities (any sort of histogram, basically).
Also add a non-thread safe version for convenience.

* Address Araq comments on https://github.com/nim-lang/Nim/pull/10072

* import at top of file and space after '#'.

* Put in a check for non-zero total weight.

* Clarify constraint on `w`.

* Rename `rand(openArray[T])` to `sample(openArray[T])` to `sample`, deprecating
old name and name new (openArray[T], openArray[U]) variants `sample`.

* Rename caller-provided state version of rand(openArray[T]) and also clean
up doc comments.

* Add test for new non-uniform array sampler.  3 sd bound makes it 99% likely
that it will still pass in the future if the random number generator changes.
We cannot both have a tight bound to check distribution *and* loose check to
ensure resilience to RNG changes.  (We cannot *guarantee* resilience, anyway.
There's always a small chance any test hits a legitimate random fluctuation.)

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