Optimize list and map packing

At least 10% and 20% respectively.

lib/msgpax/packer.ex

@@ -145,12 +145,15 @@ defimpl Msgpax.Packer, for: Map do
   end
 
   def pack(map) do
-    [format(map)] ++
-      for {key, value} <- map do
-        [@protocol.pack(key) | @protocol.pack(value)]
-      end
+    [format(map) | map |> Map.to_list() |> pack([])]
+  end
+
+  defp pack([{key, value} | rest], result) do
+    pack(rest, [@protocol.pack(key), @protocol.pack(value) | result])
   end
 
+  defp pack([], result), do: result
+
   defp format(map) do
     length = map_size(map)
 
@@ -165,12 +168,15 @@ end
 
 defimpl Msgpax.Packer, for: List do
   def pack(list) do
-    [format(list)] ++
-      for item <- list do
-        @protocol.pack(item)
-      end
+    [format(list) | list |> Enum.reverse() |> pack([])]
+  end
+
+  defp pack([item | rest], result) do
+    pack(rest, [@protocol.pack(item) | result])
   end
 
+  defp pack([], result), do: result
+
   defp format(list) do
     length = length(list)