Incompatibility with SBCL 2.2.6

[Manfred Bergmann]

Loads of error like these:

While computing the class precedence list of the class named CL-GSERVER.ACTOR:ACTOR.
The class named #:ACTOR-CELL is a forward referenced class.
The class named #:ACTOR-CELL is a direct superclass of the class named CL-GSERVER.ACTOR:ACTOR..

[Manfred Bergmann]

From the release notes it is not clear why this happens:

http://www.sbcl.org/news.html#2.2.6

[Manfred Bergmann]

Fixed with f4e48fd.

The problem was that actor class symbol was defined in actor-api but actor-cell wasn't known at that time, which is the superclass.