Address PR review

Deal with grapheme cluster bounderies correctly.
Updated docs.

distribution/lib/Standard/Base/0.0.0-dev/src/Data/Text/Extensions.enso

@@ -602,10 +602,12 @@ Text.not_empty = this.is_empty.not
 ## Inserts text value at the specified index.
 
    Arguments:
-     - index: The location in the text to inset text at (0-based). The
-       index is also allowed to be negative, then the characters are
-       counted from the end of the text, i.e. -1 will insert the text after
-       the last x.
+     - index: The position (0-based) in the text to inset text at.
+       When the index is non-negative the text is inserted before the
+       specified position.
+       When the index is negative, then the characters are counted from
+       the end of the text and the text is inserted after the specified
+       position, i.e. -1 will insert the text after the last character.
 
    ! What is a Character?
      A character is defined as an Extended Grapheme Cluster, see Unicode
@@ -615,18 +617,19 @@ Text.not_empty = this.is_empty.not
    > Example
     Insert text at a specified index
 
+         "Hello World!".insert 0 " Cruel" == " CruelHello World!"
          "Hello World!".insert 5 " Cruel" == "Hello Cruel World!"
          "Hello World!".insert -1 " Cruel" == "Hello World! Cruel"
 Text.insert : Integer -> Text -> Text ! Index_Out_Of_Bounds_Error
 Text.insert index that =
     len = this.length
     idx = if index < 0 then len + index + 1 else index
     if (idx < 0) || (idx > len) then Error.throw (Index_Out_Of_Bounds_Error index len) else
-        if idx == 0 then Text_Utils.concat that this else
-            if idx == len then Text_Utils.concat this that else
-                pre = Text_Utils.substring this 0 idx
-                post = Text_Utils.substring this idx len
-                Text_Utils.concat pre that post
+        if idx == 0 then that + this else
+            if idx == len then this + that else
+                pre = this.take (Range 0 idx)
+                post = this.take (Range idx len)
+                pre + that + post
 
 ## Returns if a character from the text at the specified index (0-based) is a
    digit (0-9).

std-bits/base/src/main/java/org/enso/base/Text_Utils.java

@@ -186,20 +186,6 @@ public static boolean contains(String string, String substring) {
     return searcher.first() != StringSearch.DONE;
   }
 
-  /**
-   * Combine strings {@code strings}
-   *
-   * @param strings the string values to be concatenated
-   * @return the resulting string
-   */
-  public static String concat(String... strings) {
-    StringBuilder sb = new StringBuilder();
-    for (String arg: strings) {
-      sb.append(arg);
-    }
-    return sb.toString();
-  }
-
   /**
    * Replaces all occurrences of {@code oldSequence} within {@code str} with {@code newSequence}.
    *

test/Tests/src/Data/Text_Spec.enso

@@ -344,18 +344,27 @@ spec =
             hello_world.insert 5 cruel . should_equal "Hello Cruel World!"
             hello_world.insert (hello_world.length - 1) cruel . should_equal "Hello World Cruel!"
             hello_world.insert hello_world.length cruel . should_equal "Hello World! Cruel"
+            txt = kshi + facepalm + accent_1
+            txt.insert 1 cruel . should_equal (kshi + cruel + facepalm + accent_1)
+            txt.insert 3 cruel . should_equal (kshi + facepalm + accent_1 + cruel)
 
         Test.specify "should report Index_Out_Of_Bounds_Error when inserting text at an invalid positive index position" <|
             hello_world.insert (hello_world.length + 1) cruel . should_fail_with Index_Out_Of_Bounds_Error
+            (kshi + facepalm + accent_1).insert 4 cruel . should_fail_with Index_Out_Of_Bounds_Error
 
         Test.specify "should insert text at a negative index position" <|
             hello_world.insert -1 cruel . should_equal "Hello World! Cruel"
             hello_world.insert -5 cruel . should_equal "Hello Wo Cruelrld!"
             hello_world.insert -(hello_world.length) cruel . should_equal "H Cruelello World!"
-            hello_world.insert -(hello_world.length + 1) cruel . should_equal "" CruelHello World!"
+            hello_world.insert -(hello_world.length + 1) cruel . should_equal " CruelHello World!"
+            txt = kshi + facepalm + accent_1
+            txt.insert -1 cruel . should_equal (txt + cruel)
+            txt.insert -(txt.length) cruel . should_equal (kshi + cruel + facepalm + accent_1)
 
         Test.specify "should report Index_Out_Of_Bounds_Error when inserting text at an invalid negative index position" <|
             hello_world.insert -(hello_world.length + 2) cruel . should_fail_with Index_Out_Of_Bounds_Error
+            txt = kshi + facepalm + accent_1
+            txt.insert -(txt.length + 2) cruel . should_fail_with Index_Out_Of_Bounds_Error
 
         Test.specify "should be able to check by index if is a digit" <|
             str = kshi + "A12" + accent_2