fix three typeof conversion issues

- type of strings are left implicit
- type of implicitly-typed params uses the type of the default value
- prevent exponential blowup for large ternary expressions

test/basic/string.v

@@ -0,0 +1,5 @@
+module top;
+    parameter FOO = "some useful string";
+    localparam BAR = "some other useful string";
+    initial $display("'%s' '%s'", FOO, BAR);
+endmodule