Rename XContentString#length() to stringLength()

Author: jordan-powers

libs/x-content/src/main/java/org/elasticsearch/xcontent/Text.java

@@ -80,7 +80,7 @@ public String string() {
     }
 
     @Override
-    public int length() {
+    public int stringLength() {
         if (length < 0) {
             length = string().length();
         }

libs/x-content/src/main/java/org/elasticsearch/xcontent/XContentString.java

@@ -25,5 +25,5 @@ public interface XContentString {
     /**
      * Returns the number of characters in the represented string.
      */
-    int length();
+    int stringLength();
 }