Wrap deftypes in eval-when form.

Without this cl-murmurhash does not build in Allegro Lisp 9.0 x64 SMP.
It seems that the problem is the defmethod specialized on
 #.(class-of (make-array 0 :element-type 'octet)) at line 415.
Allegro does not appear to have the type defined until after the file
is completely compiled, a problem I've run into a few times.