Update from version 0.73 to version 0.75.

Upstream changes:
0.75 Tue, December 31, 2008
    * Class::MOP::Class
      - A class that was made immutable and then mutable could end up
        sharing an immutable transformer object
        (Class::MOP::Immutable) with other classes, leading to all
        sorts of odd bugs. Reported by t0m. (Dave Rolsky)

0.74 Tue, December 25, 2008
    * MOP.xs
      - Add an xs implementation of Class::MOP::is_class_loaded (closes
        RT#41862). Based on a patch by Goro Fuji. (Florian Ragwitz)
      - Changed internals to make prehashing of hash keys easier and less
        error-prone. (Florian Ragwitz)
    * Class::MOP::Class
      - Fix documentation to show that around modifiers happen on both
        sides of the modified method. (Dave Rolsky)


(seb)