Add Gateway contract to make calls to external contracts (#72)

* Add jsonrpc call() method

* Introduce the Gateway contract

README.md

@@ -81,6 +81,30 @@ contract Example {
 }
 ```
 
+### Gateway
+
+Helper library to interact with contracts from other chains.
+
+#### Example usage
+
+```solidity
+import "suave-std/Gateway.sol";
+
+contract Example {
+    function example() public {
+        // query the beacon chain deposit contract
+        Gateway gateway = new Gateway("http://<jsonrpc endpoint>", address(0x00000000219ab540356cBB839Cbe05303d7705Fa));
+        DepositContract depositContract = DepositContract(address(gateway));
+
+        bytes memory count = depositContract.get_deposit_count();
+    }
+}
+
+interface DepositContract {
+    function get_deposit_count() external view returns (bytes memory);
+}
+```
+
 ### protocols/MevShare.sol
 
 Helper library to send bundle requests with the Mev-Share protocol.

src/Gateway.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.8;
+
+import "./protocols/EthJsonRPC.sol";
+import "forge-std/console.sol";
+
+contract Gateway {
+    EthJsonRPC ethjsonrpc;
+    address target;
+
+    constructor(string memory _jsonrpc, address _target) {
+        ethjsonrpc = new EthJsonRPC(_jsonrpc);
+        target = _target;
+    }
+
+    fallback() external {
+        bytes memory ret = ethjsonrpc.call(target, msg.data);
+
+        assembly {
+            let location := ret
+            let length := mload(ret)
+            return(add(location, 0x20), length)
+        }
+    }
+}

test/Gateway.t.sol

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: Unlicense
+pragma solidity ^0.8.13;
+
+import "forge-std/Test.sol";
+import "src/Gateway.sol";
+import "src/Test.sol";
+
+contract TestGateway is Test, SuaveEnabled {
+    function testGateway() public {
+        string memory endpoint = getEthJsonRPC();
+        Gateway gateway = new Gateway(endpoint, address(0x00000000219ab540356cBB839Cbe05303d7705Fa));
+        DepositContract depositContract = DepositContract(address(gateway));
+
+        bytes memory count = depositContract.get_deposit_count();
+        require(count.length > 0, "count is empty");
+    }
+
+    function getEthJsonRPC() public returns (string memory) {
+        try vm.envString("JSONRPC_ENDPOINT") returns (string memory endpoint) {
+            return endpoint;
+        } catch {
+            vm.skip(true);
+        }
+    }
+}
+
+interface DepositContract {
+    function get_deposit_count() external view returns (bytes memory);
+}