Issue 8409 - Proposal: implement arr.dup in library

http://d.puremagic.com/issues/show_bug.cgi?id=8409

This works with UFCS, and doesn't break existing codes.

<table>
<tr><td><code>arr.dup</code>, <code>arr.idup</code></td><td>built-in property is used</td></tr>
<tr><td><code>arr.dup()</code></td><td>library version is used</td></tr>
</table>

Requires: https://github.com/D-Programming-Language/dmd/pull/1110