Remove utils/core_ext and change kernel to require both utils + serializer directly

Author: omarqureshi

packages/@jsii/ruby-runtime/lib/jsii/kernel.rb

@@ -7,7 +7,8 @@
 require 'monitor'
 
 require_relative 'kernel/async'
-require_relative 'utils/core_ext'
+require_relative 'utils'
+require_relative 'serializer'
 require_relative 'kernel/api'
 require_relative 'kernel/callbacks'