make rng seed extendable

[Yneth]

Cell is not Send thus it cannot be used as struct field in async environment. I tried to implement cursor, which needs to have its' data shuffled in a deterministic manner.

example:

struct Cursor {
  idx: usize,
  buf: Vec<T>,
  rng: Rng,
  seed: u64
}

impl Cursor {
  async fn next() {
      // when idx > buf.len(), refresh, shuffle, continue
  }
}

[Yneth]

does this PR make sense to you?

[fogti]

As mentioned in #31 it might be more beneficial to get rid of interior mutability altogether and instead let the user wrap it in whatever interior mutability abstraction they want to use.