Use the unnamed prepared statement for Pgx.execute

The unnamed prepared statement is optimized differently, assuming the
query will only be executed once (which is what Pgx.execute does):

https://www.postgresql.org/docs/current/static/protocol-overview.html#PROTOCOL-QUERY-CONCEPTS

> In addition, an “unnamed” prepared statement and portal exist.
> Although these behave largely the same as named objects, operations
> on them are optimized for the case of executing a query only once
> and then discarding it, whereas operations on named objects are
> optimized on the expectation of multiple uses.

pgx/src/pgx.ml

@@ -1345,15 +1345,15 @@ module Make (Thread : IO) = struct
           fail_msg "Pgx.execute: Query returned multiple result sets but \
                     execute should only ever return one. Query was: %s" query)
     | Some params ->
-      Prepared.(with_prepare db ~query ~f:(fun s ->
+      Prepared.(with_prepare db ~name:"" ~query ~f:(fun s ->
         execute s ~params))
 
   let execute_iter ?(params=[]) db query ~f =
-    Prepared.(with_prepare db ~query ~f:(fun s ->
+    Prepared.(with_prepare db ~name:"" ~query ~f:(fun s ->
       execute_iter s ~params ~f))
 
   let execute_fold ?(params=[]) db query ~init ~f =
-    Prepared.(with_prepare db ~query ~f:(fun s ->
+    Prepared.(with_prepare db ~name:"" ~query ~f:(fun s ->
       execute_fold s ~params ~init ~f))
 
   let begin_work ?isolation ?access ?deferrable seq =