Package reservoir samples values uniformly at random from an unbounded sequence of inputs

Package reservoir samples values uniformly at random
from an unbounded sequence of inputs.

import "kr.dev/reservoir"

For instance, errors. If you have more errors than
you know what to do with, and you can't read them all
or even store them all, and you want to get a rough
idea of what is going on, here's how you could do it:

	rs := reservoir.New[error](100)
	...
	rs.Add(err) // add errors
	...
	rs.Add(err) // unlimited amount of errors
	...
	errs := make([]error, rs.Cap())
	n := rs.Read(errs)
	fmt.Println("sampled:", errs[:n])

This will print up to 100 error values, sampled
uniformly, without replacement, from the sequence
of all errors that have been added to rs so far.