In applications which use :all helpers (the default), most controllers
won't be making modifications to their _helpers module.

In CRuby this created many ICLASS objects which could cause a large
increase in memory usage in applications with many controllers and
helpers.

To avoid creating unnecessary modules this PR builds modules only when a
modification is being made: ethier by calling `helper`, `helper_method`,
or through having a default helper (one matching the controller's name)
included onto it.