Implement model for String.equals

String.equals is currently handled internally by CBMC, here we update
the model so that String.equals is supported when we move support
to models-library.

Author: codebyzeb

src/main/java/java/lang/String.java

@@ -1390,11 +1390,12 @@ public byte[] getBytes() {
      * @see  #equalsIgnoreCase(String)
      *
      * @diffblue.fullSupport
-     * @diffblue.untested
      */
     public boolean equals(Object anObject) {
-        // DIFFBLUE MODEL LIBRARY This is treated internally in CBMC
-        return CProver.nondetBoolean();
+        if (anObject instanceof String) {
+            return CProverString.equals((String)anObject, this);
+        }
+        return false;
     }
 
     /**