Lua: object timers

Expose object timers to lua scripts. For example:

   local o = obj.new("cockatrice egg");
   o:placeobj(5, 5);
   o:start_timer("hatch-egg", 3);

Available methods are:

- obj.has_timer("rot-corpse")
    returns true if object has attached timer, false otherwise.

- obj.peek_timer("hatch-egg")
    returns an integer value, which is the turn when the timer
    attached to the object would trigger. returns 0 if no such timer.

- obj.stop_timer("shrink-glob")
    stops attached timer, or if no timer type is given, stops all
    timers attached to the object.

- obj.start_timer("zombify-mon", 15)
    starts a timer with a trigger time in that many turns in the future.
    replaces any previous timer of the same type.

Valid timers are "rot-organic", "rot-corpse", "revive-mon",
"zombify-mon", "burn-obj", "hatch-egg", "fig-transform",
and "shrink-glob". Also "melt-ice" is recognized, but does nothing
to objects.

include/extern.h

@@ -1755,6 +1755,7 @@ extern void nhl_error(lua_State *, const char *) NORETURN;
 extern void lcheck_param_table(lua_State *);
 extern schar get_table_mapchr(lua_State *, const char *);
 extern schar get_table_mapchr_opt(lua_State *, const char *, schar);
+extern short nhl_get_timertype(lua_State *, int);
 extern void nhl_add_table_entry_int(lua_State *, const char *, int);
 extern void nhl_add_table_entry_char(lua_State *, const char *, char);
 extern void nhl_add_table_entry_str(lua_State *, const char *, const char *);