Move check for :crypto.hash_equals/2 to compile-time.

lib/plug/crypto.ex

@@ -6,7 +6,6 @@ defmodule Plug.Crypto do
   `Plug.Crypto.MessageEncryptor`, and `Plug.Crypto.MessageVerifier`.
   """
 
-  import Bitwise
   alias Plug.Crypto.{KeyGenerator, MessageVerifier, MessageEncryptor}
 
   @doc """
@@ -131,22 +130,25 @@ defmodule Plug.Crypto do
     byte_size(left) == byte_size(right) and crypto_hash_equals(left, right)
   end
 
-  defp crypto_hash_equals(x, y) do
-    # TODO: remove when we require OTP 25.0
-    if Code.ensure_loaded?(:crypto) and function_exported?(:crypto, :hash_equals, 2) do
+  # TODO: remove when we require OTP 25.0
+  if Code.ensure_loaded?(:crypto) and function_exported?(:crypto, :hash_equals, 2) do
+    defp crypto_hash_equals(x, y) do
       :crypto.hash_equals(x, y)
-    else
+    end
+  else
+    defp crypto_hash_equals(x, y) do
       legacy_secure_compare(x, y, 0)
     end
-  end
 
-  defp legacy_secure_compare(<<x, left::binary>>, <<y, right::binary>>, acc) do
-    xorred = bxor(x, y)
-    legacy_secure_compare(left, right, acc ||| xorred)
-  end
+    defp legacy_secure_compare(<<x, left::binary>>, <<y, right::binary>>, acc) do
+      import Bitwise
+      xorred = bxor(x, y)
+      legacy_secure_compare(left, right, acc ||| xorred)
+    end
 
-  defp legacy_secure_compare(<<>>, <<>>, acc) do
-    acc === 0
+    defp legacy_secure_compare(<<>>, <<>>, acc) do
+      acc === 0
+    end
   end
 
   @doc """