Warnings with SBCL

[Guthur]

The prog1 in defmodel causes warnings because the defclass is not as a top-level-form.

One solution would be to replace the prog1 with a progn then return the class at the end using (find-class ',class).

Cheers

[cbaggers]

This PR should help with that https://github.com/kennytilton/cells/pull/5