Fixing output of substring-fill! and vector-fill!

- fixed substring-fill! and vector-fill! to return void, reflecting the
  documented return value of unspecified value.  Also changes substring-fill!
  to use define-who instead of repeating 'substring-fill! in all the error
  messages.
    5_4.ss, 5_6.ss

original commit: 3f65788b5422693f3648a9e2fe575f464eb31ccd

LOG

@@ -758,3 +758,9 @@
     primdata.ss
 - added support for Visual Studio 2017.15.5
     wininstall/locate-vcredist.bat
+- fixed substring-fill! and vector-fill! to return void, reflecting the
+  documented return value of unspecified value.  Also changes substring-fill!
+  to use define-who instead of repeating 'substring-fill! in all the error
+  messages.
+    5_4.ss, 5_6.ss
+

s/5_4.ss

@@ -120,19 +120,19 @@
          ($string-set-immutable! v2)
          v2)])))
 
-(define substring-fill!
+(define-who substring-fill!
    (lambda (s m n c)
       (unless (mutable-string? s)
-         ($oops 'substring-fill! "~s is not a mutable string" s))
+         ($oops who "~s is not a mutable string" s))
       (unless (char? c)
-         ($oops 'substring-fill! "~s is not a character" c))
+         ($oops who "~s is not a character" c))
       (let ([k (string-length s)])
          (unless (and (fixnum? m) (fixnum? n) (fx<= 0 m n k))
-            ($oops 'substring-fill!
+            ($oops who
                    "~s and ~s are not valid start/end indices for ~s"
                    m n s))
          (do ([i m (fx+ i 1)])
-             ((fx= i n) s)
+             ((fx= i n))
              (string-set! s i c)))))
 
 (set! string-for-each

s/5_6.ss

@@ -85,7 +85,7 @@
     (unless (mutable-vector? v) ($oops who "~s is not a mutable vector" v))
     (let ([n (vector-length v)])
       (do ([i 0 (fx+ i 1)])
-          ((fx= i n) v)
+          ((fx= i n))
         (vector-set! v i obj)))))
 
 (set! fxvector->list