1.0.30.20: less DEFGENERIC clobbers FTYPE STYLE-WARNINGS

1.0.30.20: less DEFGENERIC clobbers FTYPE STYLE-WARNINGS

 * Remove the declamation from DESCRIBE-OBJECT.

 * Make SBCL warn only if the new type is more general than the old
   type.

 * In NOTE-GF-SIGNATURE, use the existing GF lambda-list if the user
   didn't provide one to ENSURE-GENERIC-FUNCTION. This allows us to
   deduce sufficiently good types for condition slot readers from the
   lambda-list to elide the warning.