simple-string-p object ⇒  generalized-boolean
| object | an object. | 
| generalized-boolean | a generalized boolean. | 
Returns true if object is of type simple-string; otherwise, returns false.
 (simple-string-p "aaaaaa") ⇒  true
 (simple-string-p (make-array 6 
                              :element-type 'character 
                              :fill-pointer t)) ⇒  false
 (simple-string-p object) ≡ (typep object 'simple-string)