support structs of integers

test/basic/struct_integer.sv

@@ -0,0 +1,7 @@
+module top;
+    typedef struct packed {
+        integer a, b, c;
+    } S;
+    S s = '{a: 1, b: 2, c: 3};
+    initial #1 $display("%b %b %b %b", s, s.a, s.b, s.c);
+endmodule