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/fuzz/fuzz_helper/Cargo.toml

@@ -9,6 +9,5 @@ edition = "2018"
 arrayref = "0.3.6"
 opensk = { path = "../..", features = ["fuzz"] }
 crypto = { path = "../../../crypto", features = ['std'] }
-rng256 = { path = "../../../rng256", features = ['std'] }
 sk-cbor = { path = "../../../cbor" }
 arbitrary = { version = "0.4.7", features = ["derive"] }