Eager load the constants

In a thread-based backend like Sidekiq, there is a possibility that
autoload may occur simultaneously in multiple threads, and as a result,
it is presumed that an error may be caused by contention of autoload.

In order to avoid the above issue, eager load the constants on boot.
Maybe fixes #101

lib/global_id.rb

@@ -1,9 +1,14 @@
 require 'global_id/global_id'
+require 'active_support'
 
 autoload :SignedGlobalID, 'global_id/signed_global_id'
 
 class GlobalID
-  autoload :Locator, 'global_id/locator'
-  autoload :Identification, 'global_id/identification'
-  autoload :Verifier, 'global_id/verifier'
+  extend ActiveSupport::Autoload
+
+  eager_autoload do
+    autoload :Locator, 'global_id/locator'
+    autoload :Identification, 'global_id/identification'
+    autoload :Verifier, 'global_id/verifier'
+  end
 end