Yul: Implement memory struct allocation

test/libsolidity/semanticTests/structs/nested_struct_allocation.sol

@@ -0,0 +1,18 @@
+contract C {
+  struct I {
+    uint b;
+    uint c;
+  }
+  struct S {
+    I a;
+  }
+
+  function f() external returns (uint) {
+    S memory s = S(I(1,2));
+    return s.a.b;
+  }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 1