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

run_desktop_tests.sh

@@ -26,9 +26,6 @@ cd ../..
 cd libraries/crypto
 cargo fmt -- --check
 cd ../..
-cd libraries/rng256
-cargo fmt -- --check
-cd ../..
 cd libraries/persistent_store
 cargo fmt -- --check
 cd ../..
@@ -111,9 +108,6 @@ then
   cd libraries/cbor
   cargo test --release
   cd ../..
-  cd libraries/rng256
-  cargo test --release --features std
-  cd ../..
   cd libraries/persistent_store
   cargo test --release --features std
   cd ../..
@@ -124,9 +118,6 @@ then
   cd libraries/cbor
   cargo test
   cd ../..
-  cd libraries/rng256
-  cargo test --features std
-  cd ../..
   cd libraries/persistent_store
   cargo test --features std
   cd ../..