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/crypto/Cargo.toml

@@ -10,7 +10,6 @@ license = "Apache-2.0"
 edition = "2018"
 
 [dependencies]
-rng256 = { path = "../rng256" }
 arrayref = "0.3.6"
 subtle = { version = "2.2.3", default-features = false, features = ["nightly"] }
 byteorder = { version = "1", default-features = false }
@@ -20,7 +19,8 @@ untrusted = { version = "0.7.0", optional = true }
 serde = { version = "1.0", optional = true, features = ["derive"] }
 serde_json = { version = "=1.0.69", optional = true }
 regex = { version = "1", optional = true }
+rand_core = "0.6.4"
 
 [features]
-std = ["hex", "ring", "rng256/std", "untrusted", "serde", "serde_json", "regex"]
+std = ["hex", "ring", "untrusted", "serde", "serde_json", "regex", "rand_core/getrandom"]
 with_ctap1 = []