Add missing reverse-list-accumulator for SRFI 158 (#48)

Also fixes the description of list-accumulator, which was using the
wording for reverse-list-accumulator.

Co-authored-by: Andrew Whatson <whatson@tailcall.au>

types/srfi.158.scm

@@ -324,6 +324,11 @@ The signature of the unfold procedure is (unfold stop? mapper successor seed arg
     (return (lambda (el) (or undefined integer?))))
   (desc . "Returns an accumulator that, when invoked on an object, adds 1 to a count inside the accumulator and returns an unspecified value. However, if an end-of-file object is passed, the accumulator returns the count. "))
  ((name . "list-accumulator")
+  (signature lambda () procedure?)
+  (subsigs
+    (return (lambda (el) (or undefined list?))))
+  (desc . "Returns an accumulator that, when invoked on an object, adds that object to a list inside the accumulator in order of accumulation and returns an unspecified value. However, if an end-of-file object is passed, the accumulator returns the list."))
+ ((name . "reverse-list-accumulator")
   (signature lambda () procedure?)
   (subsigs
     (return (lambda (el) (or undefined list?))))