Seedable Step RNG

[willow-iam]

Background

What is your motivation?
I have a struct with a generic which implements SeedableRNG because it wants to store an RNG state.
What type of application is this?
Data structure: Multiset with random(rng-based) pop function.

Feature request

Seedable Step RNG: A version of StepRNG that implements the SeedableRNG trait, for easy testing.