Move sampling methods to Enumerable

[HertzDevil]

Resolves #10118. There are two caveats here:

• Since Indexable's optimization calls the RNG exactly once, it will produce different results than types that aren't Indexable, most notably the collections' own external iterators:

  ary = [1, 2, 3, 4, 5]
  ary.sample(Random.new(1))      # => 3
  ary.each.sample(Random.new(1)) # => 1

• Technically Range(Float, Float)#sample(random) doesn't mean the same thing as the other sampling methods, because in this case it represents a continuous uniform distribution rather than a discrete collection of items. I allowed it anyway for consistency with Range(Int, Int). The #sample(n, random) overload still won't work as it requires Float#succ (c.f. Float-based ranges #9339).

[asterite]

Oh, needs a conflict resolution, but after that I'll merge it. Thank you!

[bcardiff]

Looks good. I would suggest to mention somewhere in docs about reservoir sampling. Currently I need to get into the issue discussion to understand why the equal probability statement holds.

[asterite]

@bcardiff Do you mean add a doc comment, or just an internal comment about reservoir sampling? The old code was also using it but it wasn't mentioned. I would mention it as an internal comment in case it needs to be updated, but I wouldn't expose this fact. You don't need to know this to use the method; if you are curious you can dig into the code; there are no alternatives; and reading "reservoir sampling" might confuse users more than help them (for instance, Ruby docs never mention algorithms).

[bcardiff]

I was thinking of internal docs. I missed that the same algorithm was used already.

[HertzDevil]

Thinking if we could implement Algorithm L on Indexable: https://carc.in/#/r/a975

That algorithm seems to require that Random.rand return a sample in (0, 1) instead of [0, 1). I don't know what would happen when it actually returns zero...

[oprypin]

If you're worried about it returning zero, then there's always the strategy of just trying again if it gives you zero. It's definitely correct and not a bad strategy overall, it's already being used elsewhere.

[asterite]

Maybe the optimization can be undone and moved to a separate PR? Because now this needs 2 reviews again.

[straight-shoota]

Thank you