Add custom RandomNumberGenerator source to Int.init?(randomBelow:)

[knothed]

This allows users to provide the RNG of their choice for creating pseudo-random integers.

I.e., instead of

let values = 100.timesMake { Int(randomBelow: 10) }

users could write:

var xoshiro = Xoshiro() // Some third-party RNG
let values = 100.timesMake { Int(randomBelow: 10, using: &xoshiro) }

[Jeehut]

Makes sense, merging. Thank you very much!