improved handling of string parameters

- variable-size string parameter conversion restricted to modules which
  depend on the size of the string parameter
- string localparams are typed as appropriately sized vectors
- remove ordered parameter binding logic from string param conversion

test/basic/string_param_plain_tb.v

@@ -0,0 +1,4 @@
+module top;
+    mod #("FOO") m1();
+    mod #("BAR") m2();
+endmodule