fix random

should address #8 (closed)

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