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/ctap/data_formats.rs

@@ -1232,13 +1232,13 @@ mod test {
     use super::*;
     use crate::api::crypto::ecdh::PublicKey as _;
     use crate::api::crypto::ecdsa::PublicKey as _;
+    use crate::api::rng::Rng;
     use crate::env::test::TestEnv;
-    use crate::env::{EcdhPk, EcdsaPk};
+    use crate::env::{EcdhPk, EcdsaPk, Env};
     use cbor::{
         cbor_array, cbor_bool, cbor_bytes, cbor_bytes_lit, cbor_false, cbor_int, cbor_null,
         cbor_text, cbor_unsigned,
     };
-    use rng256::Rng256;
 
     #[test]
     fn test_extract_unsigned() {