patch 9.0.1515: reverse() does not work for a String

Problem:    reverse() does not work for a String.
Solution:   Implement reverse() for a String. (Yegappan Lakshmanan,
            closes #12179)

runtime/doc/builtin.txt

@@ -484,7 +484,8 @@ rename({from}, {to})		Number	rename (move) file from {from} to {to}
 repeat({expr}, {count})		List/Blob/String
 					repeat {expr} {count} times
 resolve({filename})		String	get filename a shortcut points to
-reverse({list})			List	reverse {list} in-place
+reverse({obj})			List/Blob/String
+					reverse {obj}
 round({expr})			Float	round off {expr}
 rubyeval({expr})		any	evaluate |Ruby| expression
 screenattr({row}, {col})	Number	attribute at screen position
@@ -7404,11 +7405,13 @@ resolve({filename})					*resolve()* *E655*
 			GetName()->resolve()
 
 reverse({object})					*reverse()*
-		Reverse the order of items in {object} in-place.
-		{object} can be a |List| or a |Blob|.
-		Returns {object}.
-		Returns zero if {object} is not a List or a Blob.
-		If you want an object to remain unmodified make a copy first: >
+		Reverse the order of items in {object}.  {object} can be a
+		|List|, a |Blob| or a |String|.  For a List and a Blob the
+		items are reversed in-place and {object} is returned.
+		For a String a new String is returned.
+		Returns zero if {object} is not a List, Blob or a String.
+		If you want a List or Blob to remain unmodified make a copy
+		first: >
 			:let revlist = reverse(copy(mylist))
 <		Can also be used as a |method|: >
 			mylist->reverse()