should address #8 

## changelog
- move the internal `rng` to an argument of type `R: RngCore` for the following functions
  - `recode` in `lib.rs`
  - `linalg::Matrix::random`
  - `generate_random_setup` in `main.rs`
- make sure
  - `ark_std::test_rng` is only used in tests modules
  - `rand::thread_rng` is used in benchmarks, examples and `main.rs`