Replaces Rng256 with new Rng API (#612)

* Replaces the Rng256 with RngCore from rand_core

The old trait was designed with our software crypto in mind. We should
use a more standard API going forward.

- Removes libraries/rng256/
- Ports libraries/crypto/ to rand_core
- Moves the used RNG trait to api/

* Use StdRng directy in TestEnv

libraries/opensk/src/env/mod.rs

@@ -21,10 +21,10 @@ use crate::api::crypto::Crypto;
 use crate::api::customization::Customization;
 use crate::api::firmware_protection::FirmwareProtection;
 use crate::api::key_store::KeyStore;
+use crate::api::rng::Rng;
 use crate::api::upgrade_storage::UpgradeStorage;
 use crate::api::user_presence::UserPresence;
 use persistent_store::{Storage, Store};
-use rng256::Rng256;
 
 #[cfg(feature = "std")]
 pub mod test;
@@ -40,7 +40,7 @@ pub type Hkdf<E> = <<E as Env>::Crypto as Crypto>::Hkdf256;
 
 /// Describes what CTAP needs to function.
 pub trait Env {
-    type Rng: Rng256;
+    type Rng: Rng;
     type UserPresence: UserPresence;
     type Storage: Storage;
     type KeyStore: KeyStore;