support nested interfaces

test/basic/interface_nested.sv

@@ -0,0 +1,15 @@
+module top;
+    logic x = 1;
+    foo f(x);
+endmodule
+
+interface foo(input logic x);
+    bar a(x);
+    bar b(~x);
+endinterface
+
+interface bar(input logic x);
+    initial begin
+        $display("bar got %b", x);
+    end
+endinterface