Add integration test for deleting struct (#459)

Differentially tests the behavior of deleting a struct with a member
that do not occupy the full 32-byte slot. From [this
issue](https://github.com/paritytech/contract-issues/issues/118) it was
unclear whether the `delete` was expected to reset only the bytes
occupied or the entire slot. The current behavior of resetting the
entire slot is expected.

crates/integration/contracts/StructDeleteStorage.sol

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8;
+
+/* runner.json
+{
+    "differential": true,
+    "actions": [
+        {
+            "Instantiate": {
+                "code": {
+                    "Solidity": {
+                        "contract": "StructDeleteStorage"
+                    }
+                }
+            }
+        },
+        {
+            "Call": {
+                "dest": {
+                    "Instantiated": 0
+                },
+                "data": "e70c830d"
+            }
+        }
+    ]
+}
+*/
+
+contract StructDeleteStorage {
+    struct S {
+        uint8 a;
+    }
+    S s;
+
+    function store_and_delete() public returns (uint256 r1) {
+        assembly {
+            sstore(s.slot, 0xffffff)
+        }
+        delete s;
+        assembly {
+            r1 := sload(s.slot)
+        }
+    }
+}