Given there's a Random API in the stdlib, that generates all random primitives, this commit adds the possibility to generate random unsigned integers.
It uses the same implementation from Random.nextInt, nextLong and nextBytes, but uses the appropriate conversions from a signed type to an unsigned type.
The use of the same API guarantees that the distribution is uniform, but this commit adds some unit test to enforce that necessity.
Fixes #KT-25570
Sergey Bogolepov