sampling arbitrary iterables

[bakkot]

This has Random.pickFromList, which is a good and useful function when your list is of known length (though a bad name). But when it's not, you can still usefully sample from it (although it's O(n) to do so). Should we expose that functionality?

[tabatkins]

Yeah, some ability to reservoir sample would be interesting (and very non-trivial to do by hand!)