Correct type declarations

cl/chronometrist.org

@@ -1272,14 +1272,14 @@ These can be implemented in terms of the minimal protocol above.
 (defclass file-backend-mixin ()
   ((file :initform nil
          :initarg :file
-         :type 'pathname
+         :type (or pathname null)
          :documentation
          "Pathname for backend file, without extension. Do not access this
          directly - use `backend-file' instead.")
    (extension :initform nil
               :initarg :extension
               :accessor backend-ext
-              :type 'string
+              :type string
               :documentation
               "Extension of backend file.")
    (hash-table :initform (make-hash-table-1)