feat: add BeaconRNG With blockhash fallback

Author: shalzz

contracts/standard/rng/BeaconRNG.sol

@@ -1,5 +1,5 @@
  /**
- *  @authors: [@shalzz
+ *  @authors: [@shalzz]
  *  @reviewers: []
  *  @auditors: []
  *  @bounties: []

contracts/standard/rng/BeaconRNGFallback.sol

@@ -0,0 +1,61 @@
+ /**
+ *  @authors: [@shalzz]
+ *  @reviewers: []
+ *  @auditors: []
+ *  @bounties: []
+ *  @deployments: []
+ */
+
+pragma solidity ^0.4.15;
+
+import "./RNG.sol";
+
+/**
+ *  @title Random Number Generator using beacon chain random opcode
+ */
+contract BeaconRNGFallBack is RNG {
+
+    RNG public beaconRNG;
+    RNG public blockhashRNG;
+
+    /** @dev Constructor.
+     * @param _beaconRNG The beacon chain RNG deployed contract address
+     * @param _blockhashRNG The blockhash RNG deployed contract address
+     */
+    constructor(RNG _beaconRNG, RNG _blockhashRNG) public {
+        beaconRNG = _beaconRNG;
+        blockhashRNG = _blockhashRNG;
+    }
+
+    /**
+     * @dev Request a random number.
+     * @dev Since the beacon chain randomness is not related to a block
+     * we can call ahead its getRN function to check if the PoS merge has happened or not.
+     *  
+     * @param _block Block linked to the request.
+     */
+    function requestRN(uint _block) public payable {
+        uint RN = beaconRNG.getRN(_block);
+
+        if (RN == 0) {
+            blockhashRNG.contribute(_block);
+        } else {
+            beaconRNG.contribute(_block);
+        }
+    }
+
+    /**
+     * @dev Get the random number.
+     * @param _block Block the random number is linked to.
+     * @return RN Random Number. If the number is not ready or has not been required 0 instead.
+     */
+    function getRN(uint _block) public returns (uint RN) {
+        RN = beaconRNG.getRN(_block);
+
+        // if beacon chain randomness is zero
+        // fallback to blockhash RNG
+        if (RN == 0) {
+            RN = blockhashRNG.getRN(_block);
+        }
+    }
+}