Implement nqp::replace op.

Author: jnthn

src/vm/jvm/runtime/org/perl6/nqp/runtime/Ops.java

@@ -2540,6 +2540,10 @@ public static String flip(String str) {
         return new StringBuffer(str).reverse().toString();
     }
 
+    public static String replace(String str, long offset, long count, String repl) {
+        return new StringBuffer(str).replace((int)offset, (int)(offset + count), repl).toString();
+    }
+    
     /* Brute force, but not normally needed for most programs. */
     private static HashMap<String, Character> cpNameMap;
     public static long codepointfromname(String name) {