Register 'utf8' Lua package and make it available to users

'utf8' is a new Lua5.3 package https://www.lua.org/manual/5.3/manual.html#6.5

docs/html/lua_parser.html

@@ -15,6 +15,7 @@ <h1>API</h1>
 	<li><span class="cmd">math</span> is defined.</li>
 	<li><span class="cmd">debug</span> is defined.</li>
 	<li><span class="cmd">io</span> is defined.</li>
+	<li><span class="cmd">utf8</span> is defined.</li>
 </ul>
 <p>Tup-specific functions are provided in the <span class="cmd">tup</span> table.  In the following documentation, <dfn>PROCESSING</dfn> refers to the directory Tup is currently processing, and <dfn>RUNNING</dfn> refers to the directory of the build definition file Tup is currently running.  When Tup enters <span class="filename">Tupfile.lua</span>, PROCESSING and RUNNING are the same.  When running <span class="filename">Tuprules.lua</span> or an explicitly included file, PROCESSING AND RUNNING may be different.</p>
 <dl>

src/tup/luaparser.c

@@ -778,6 +778,7 @@ int parse_lua_tupfile(struct tupfile *tf, struct buf *b, const char *name)
 		luaL_requiref(ls, LUA_MATHLIBNAME, luaopen_math, 1); lua_pop(ls, 1);
 		luaL_requiref(ls, LUA_DBLIBNAME, luaopen_debug, 1); lua_pop(ls, 1);
 		luaL_requiref(ls, LUA_IOLIBNAME, luaopen_io, 1); lua_pop(ls, 1);
+		luaL_requiref(ls, LUA_UTF8LIBNAME, luaopen_utf8, 1); lua_pop(ls, 1);
 		lua_pushnil(ls); lua_setglobal(ls, "dofile");
 		lua_pushnil(ls); lua_setglobal(ls, "loadfile");
 		lua_pushnil(ls); lua_setglobal(ls, "load");