feat: add a Beacon Chain RNG

Author: shalzz

contracts/standard/rng/BeaconRNG.sol

@@ -0,0 +1,38 @@
+ /**
+ *  @authors: [@shalzz
+ *  @reviewers: []
+ *  @auditors: []
+ *  @bounties: []
+ *  @deployments: []
+ */
+
+pragma solidity ^0.4.15;
+
+import "./RNG.sol";
+
+/**
+ *  @title Random Number Generator using beacon chain random opcode
+ */
+contract BeaconRNG is RNG {
+
+    /**
+     * @dev Since we don't really need to incentivise requesting the beacon chain randomness,
+     * this is a stub implementation required for backwards compatibility with the
+     * RNG interface.
+     * @notice All the ETH sent here will be lost forever.
+     * @param _block Block the random number is linked to.
+     */
+    function contribute(uint _block) public payable {}
+
+
+    /** @dev Return the random number from the PoS randomness beacon.
+     *  @param _block Block the random number is linked to.
+     *  @return RN Random Number. If the PoS upgrade defined by EIP-3675
+     *          has not yet executed 0 instead.
+     */
+    function getRN(uint _block) public returns (uint RN) {
+        if (block.difficulty <= 2**64)
+            return 0;
+        return block.difficulty;
+    }
+}