added memset_bytes, fixed some stack comments, fixed splitby_char and finally implemented splitby_str so splitby if now fully functional

tests/str.expected

@@ -1,3 +1,13 @@
 1
 0
 hello world hello world hello world hello world hello world
+ hello
+hello 
+ hello 
+hello 
+o
+d he
+o wor
+d he
+o wor
+he

tests/str.sl

@@ -14,4 +14,11 @@ word main
     2dup
     puts
     free
+
+    "hello world hello" "world" splitby
+    for puts end
+    "hello world hello world" "world" splitby
+    for puts end
+    "hello world hello world hello" "l" splitby
+    for puts end
 end