Contract calls (#19)

crates/integration/contracts/Call.sol

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8;
+
+contract Call {
+    function value_transfer(address payable destination) public payable {
+        destination.transfer(msg.value);
+    }
+
+    function echo(bytes memory payload) public pure returns (bytes memory) {
+        return payload;
+    }
+
+    function call(
+        address callee,
+        bytes memory payload
+    ) public pure returns (bytes memory) {
+        return Call(callee).echo(payload);
+    }
+}