Refactor warnings system.

Create four warning classes to handle a particular set of warnings:

- Warning::Basic -- the usual warnings :tag, :link, :global, ...
- Warning::Nodoc -- the :nodoc warning type together with its
  parameters.
- Warnings::Deprecated -- the old :no_doc* warnings which delegate to
  the new :nodoc warning.
- Warning::All -- composite warning that allows enabling/disabling all
  warning types together.

Then inside Warning::Registry delegate #set and #enabled? calls to these
warnings.