implement immutable variables codegen (#70)

crates/integration/contracts/Immutables.sol

@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8;
+
+/* runner.json
+{
+  "differential": true,
+  "actions": [
+    {
+      "Upload": {
+        "code": {
+          "Solidity": {
+            "contract": "ImmutablesTester"
+          }
+        }
+      }
+    },
+    {
+      "Instantiate": {
+        "code": {
+          "Solidity": {
+            "contract": "Immutables"
+          }
+        }
+      }
+    },
+    {
+      "Call": {
+        "dest": {
+          "Instantiated": 0
+        }
+      }
+    }
+  ]
+}
+*/
+
+contract ImmutablesTester {
+    // Read should work in the runtime code
+    uint public immutable foo;
+    // Read should work in the runtime code
+    uint public immutable bar;
+    // Read should work in the runtime code
+    uint public immutable zoo;
+
+    // Assign and read should work in the constructor
+    constructor(uint _foo) payable {
+        foo = _foo;
+        bar = foo + 1;
+        zoo = bar + 2;
+
+        assert(zoo == _foo + 3);
+    }
+}
+
+contract Immutables {
+    fallback() external {
+        ImmutablesTester tester = new ImmutablesTester(127);
+
+        assert(tester.foo() == 127);
+        assert(tester.bar() == tester.foo() + 1);
+        assert(tester.zoo() == tester.bar() + 2);
+    }
+}