Refactors the extensibility mechanism, by allowing a single call to the ExtensibilityHelper.
Gets rid of the ExtensibilityHelper.ExtractValue() method, since the value is now exposed by the runnableCheck.
Fixes the documentation of the ExtensibilityHelper class.
Renames IRunnableCheck to ICheckForExtensibilty, IRunnableStructCheck to IStructCheckForExtensibility, ICheckRunner to IRunnableCheck, etc.