Implements a Perl 5 native version of what the Perl 6 symbol export mechanism
will look like.

It's very straightforward:

    * If you want a subroutine to be capable of being exported (when
      explicitly requested in the use arguments), you mark it with the
      :Export attribute.
    * If you want a subroutine to be automatically exported when the module
      is used (without specific overriding arguments), you mark it with the
      :Export(:DEFAULT) attribute.
    * If you want a subroutine to be automatically exported when the module
      is used (even if the user specifies overriding arguments), you mark it
      with the :Export(:MANDATORY) attribute.
    * If the subroutine should also be exported when particular export
      groups are requested, you add the names of those export groups to the
      attribute's argument list.

That's it.