Parse Unicode string escape \u{XX...}.

Thanks to drbo.

doc/changes.html

@@ -86,6 +86,7 @@ Please take a look at the commit history for more details.
 <li>Add <tt>LJ_GC64</tt> mode: 64 bit GC object references (really: 47 bit). Interpreter-only for now.</li>
 <li>Add <tt>LJ_FR2</tt> mode: Two-slot frame info. Required by <tt>LJ_GC64</tt> mode.</li>
 <li>Add <tt>table.new()</tt> and <tt>table.clear()</tt>.</li>
+<li>Parse Unicode escape <tt>'\u{XX...}'</tt> in string literals.</li>
 <li>Parse binary number literals (<tt>0bxxx</tt>).</li>
 </ul></li>
 <li>Improvements to the JIT compiler:

doc/extensions.html

@@ -344,6 +344,13 @@ Lua 5.1, which prevents implementing features that would otherwise
 break the Lua/C API and ABI (e.g. <tt>_ENV</tt>).
 </p>
 
+<h2 id="lua53">Extensions from Lua 5.3</h2>
+<p>
+LuaJIT supports some extensions from Lua&nbsp;5.3:
+<ul>
+<li>Unicode escape <tt>'\u{XX...}'</tt> embeds the UTF-8 encoding in string literals.</li>
+</ul>
+
 <h2 id="exceptions">C++ Exception Interoperability</h2>
 <p>
 LuaJIT has built-in support for interoperating with C++&nbsp;exceptions.