Move implementation to own file

Prefactoring, from  TDDAYMI to imperative shell. In the next step we
need the implementation from the functional core in the imperative
shell, to pass in the environment variable.

test/fun_core/basic_signup_whitelist_test.exs

@@ -1,23 +1,6 @@
 defmodule FunCore.BasicSignupWhitelistTest do
   use ExUnit.Case
-
-  defp normalize(email) do
-    email |> String.trim() |> String.downcase()
-  end
-
-  defp addresses_as_list(addresses_str) do
-    addresses_str
-    |> String.split(",")
-    |> Enum.map(&normalize/1)
-  end
-
-  def mail_whitelisted_fun(signups_allowed, email_received) do
-    case signups_allowed do
-      nil -> false
-      "*" -> true
-      list_str -> normalize(email_received) in addresses_as_list(list_str)
-    end
-  end
+  import FunCore.BasicSignupWhitelist
 
   test "addresses_as_list" do
     assert addresses_as_list("joe@example.com, jane@example.com") == [