Minor pickle optimization for Map and list (#7812)

* Minor pickle optimization for Map and list

* Minor feedback changes

* quick style change

src/fsharp/TastPickle.fs

@@ -581,8 +581,7 @@ let u_array_ext extraf f st =
     extraItem, arr
 
 let u_list_core f n st =
-    [ for _ in 1..n do
-         yield f st ]
+    List.init n (fun _ -> f st)
 
 let u_list f st =
     let n = u_int st
@@ -1305,11 +1304,23 @@ let u_ILInstr st =
 // Pickle/unpickle for F# types and module signatures
 //---------------------------------------------------------------------------
 
-let p_Map pk pv = p_wrap Map.toList (p_list (p_tup2 pk pv))
+let p_Map_core pk pv xs st =
+    xs |> Map.iter (fun k v -> pk k st; pv v st)
+
+let p_Map pk pv x st =
+    p_int (Map.count x) st
+    p_Map_core pk pv x st
+
 let p_qlist pv = p_wrap QueueList.toList (p_list pv)
 let p_namemap p = p_Map p_string p
 
-let u_Map uk uv = u_wrap Map.ofList (u_list (u_tup2 uk uv))
+let u_Map_core uk uv n st =
+    Map.ofSeq (seq { for _ in 1..n -> (uk st, uv st) })
+
+let u_Map uk uv st = 
+    let n = u_int st
+    u_Map_core uk uv n st
+
 let u_qlist uv = u_wrap QueueList.ofList (u_list uv)
 let u_namemap u = u_Map u_string u