Rtop: make show directive work on remapped operators

[anmonteiro]

Fixes #1766

Since I was touching rtop in #2146, might as well fix this one too.

Before:

Reason # #show (^);
let ( ++ ): (string, string) => string;
Reason # #show (++);
Unknown element.
Reason # #toggle_syntax;
OCaml # #show (^);;
let ( ++ ): (string, string) => string;
OCaml # #show (++);;
Unknown element.
OCaml # #toggle_syntax;;
Reason # #show (++);
Unknown element.

After:

Reason # #show (^);
external ( ^ ): ref('a) => 'a = "%field0";
Reason # #show (++);
let ( ++ ): (string, string) => string;
Reason # #toggle_syntax;
OCaml # #show (^);;
val ( ^ ) : string -> string -> string
OCaml # #show (++);;
Unknown element.
OCaml # #toggle_syntax;;
Reason # #show (++);
let ( ++ ): (string, string) => string;