Introduce efficient Hash-based RNG streamer (#4169)

* Introduce efficient Hash-based RNG streamer

* Loop in the case of being in bias space.

* Fix tests

* Minor cleanup

substrate/primitives/sr-primitives/src/lib.rs

@@ -52,6 +52,7 @@ pub mod offchain;
 pub mod testing;
 pub mod traits;
 pub mod transaction_validity;
+pub mod random_number_generator;
 
 /// Re-export these since they're only "kind of" generic.
 pub use generic::{DigestItem, Digest};
@@ -70,6 +71,8 @@ pub use arithmetic::helpers_128bit;
 /// Re-export big_uint stuff.
 pub use arithmetic::biguint;
 
+pub use random_number_generator::RandomNumberGenerator;
+
 /// An abstraction over justification for a block's validity under a consensus algorithm.
 ///
 /// Essentially a finality proof. The exact formulation will vary between consensus