implement the coinbase opcode (#179)

Signed-off-by: xermicus <cyrill@parity.io>

crates/integration/contracts/Coinbase.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8;
+
+/* runner.json
+{
+    "differential": true,
+    "actions": [
+        {
+            "Instantiate": {
+                "code": {
+                    "Solidity": {
+                        "contract": "Coinbase"
+                    }
+                }
+            }
+        }
+    ]
+}
+*/
+
+contract Coinbase {
+    constructor() payable {
+        address coinbase = address(0xFFfFfFffFFfffFFfFFfFFFFFffFFFffffFfFFFfF);
+        assert(block.coinbase == coinbase);
+    }
+}