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.sv

@@ -0,0 +1,10 @@
+module other;
+    parameter STR = "missing";
+    initial $display("other: STR=%s $bits(STR)=%0d", STR, $bits(STR));
+endmodule
+
+module mod;
+    parameter STR = "missing";
+    initial $display("mod: STR=%s", STR);
+    other #("HI") m();
+endmodule