feat: UsingWitnetRandomness abstract contract

contracts/apps/UsingWitnetRandomness.sol

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.7.0 <0.9.0;
+pragma experimental ABIEncoderV2;
+
+import "../WitnetRandomness.sol";
+
+/// @title The UsingWitnetRandomness contract
+/// @dev Contracts willing to interact with WitnetRandomness appliance should just inherit from this contract.
+/// @author The Witnet Foundation.
+abstract contract UsingWitnetRandomness
+    is
+        IWitnetOracleEvents,
+        IWitnetRandomnessEvents
+{
+    WitnetOracle immutable public witnet;
+    WitnetRandomness immutable public _RNG;
+
+    constructor(WitnetRandomness _witnetRandomness) {
+        require(
+            address(_witnetRandomness).code.length > 0
+                && _witnetRandomness.specs() == type(WitnetRandomness).interfaceId,
+            "UsingWitnetRandomness: uncompliant WitnetRandomness appliance"
+        );
+        _RNG = _witnetRandomness;
+        witnet = _RNG.witnet();
+    }
+
+}
+