Fix use Bitwise warning (#296)

Author: thanabodee-c

lib/protobuf/wire/varint.ex

@@ -43,7 +43,7 @@ defmodule Protobuf.Wire.Varint do
       iex> Protobuf.Wire.Varint.encode(1_234_567)
       [<<135>>, <<173>>, 75]
   """
-  use Bitwise
+  import Bitwise
 
   @max_bits 64
   @mask64 bsl(1, @max_bits) - 1

lib/protobuf/wire/zigzag.ex

@@ -1,7 +1,7 @@
 defmodule Protobuf.Wire.Zigzag do
   @moduledoc false
 
-  use Bitwise, skip_operators: true
+  import Bitwise
 
   @spec encode(integer) :: integer
   def encode(n) when n >= 0, do: n * 2