Add tests.

test/libyul/objectCompiler/subSubObject.yul

@@ -0,0 +1,39 @@
+object "a" {
+  code {}
+  // Unreferenced data is not added to the assembled bytecode.
+  data "str" "Hello, World!"
+  object "sub" {
+    code { sstore(0, 1) }
+    object "subsub" {
+      code { sstore(2, 3) }
+      data "str" hex"123456"
+    }
+  }
+}
+// ----
+// Assembly:
+// stop
+// data_acaf3289d7b601cbd114fb36c4d29c85bbfd5e133f14cb355c3fd8d99367964f 48656c6c6f2c20576f726c6421
+//
+// sub_0: assembly {
+//         /* "source":153:154   */
+//       0x01
+//         /* "source":150:151   */
+//       0x00
+//         /* "source":143:155   */
+//       sstore
+//     stop
+//
+//     sub_0: assembly {
+//             /* "source":203:204   */
+//           0x03
+//             /* "source":200:201   */
+//           0x02
+//             /* "source":193:205   */
+//           sstore
+//         stop
+//         data_6adf031833174bbe4c85eafe59ddb54e6584648c2c962c6f94791ab49caa0ad4 123456
+//     }
+// }
+// Bytecode: fe
+// Opcodes: INVALID