interface conversion supports positional port bindings

- also fixes an issue where system tasks were inadvertently prefixed
  during the interface conversion

test/basic/interface_infer.sv

@@ -0,0 +1,14 @@
+interface Interface;
+    logic x;
+    modport Modport(
+        input x
+    );
+    initial $display("Interface", x);
+endinterface
+module Module(Interface.Modport foo);
+    initial $display("Module", foo.x);
+endmodule
+module top;
+    Interface i();
+    Module m(i);
+endmodule

test/basic/interface_infer.v

@@ -0,0 +1,8 @@
+module Module(input wire x);
+    initial $display("Module", x);
+endmodule
+module top;
+    wire i_x;
+    initial $display("Interface", i_x);
+    Module m(.x(i_x));
+endmodule