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

.github/workflows/rng256_test.yml

@@ -1,39 +0,0 @@
----
-name: RNG library tests
-on:
-  push:
-    paths:
-      - 'libraries/rng256/**'
-  pull_request:
-    types: [opened, synchronize, reopened]
-    paths:
-      - 'libraries/rng256/**'
-
-jobs:
-  rng256_test:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: "true"
-      - name: Install Rust toolchain
-        run: rustup show
-      - uses: actions/setup-python@v1
-        with:
-          python-version: 3.7
-      - name: Install Python dependencies
-        run: python -m pip install --upgrade pip setuptools wheel
-      - name: Set up OpenSK
-        run: ./setup.sh
-
-      - name: Unit testing of rng256library (release mode)
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --manifest-path libraries/rng256/Cargo.toml --release --features std
-
-      - name: Unit testing of rng256 library (debug mode)
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --manifest-path libraries/rng256/Cargo.toml --features std