new function 'lua_isyieldable' (and 'coroutine.isyieldable')

ldo.c

@@ -1,5 +1,5 @@
 /*
-** $Id: ldo.c,v 2.114 2014/02/26 15:27:56 roberto Exp roberto $
+** $Id: ldo.c,v 2.115 2014/03/21 13:52:33 roberto Exp roberto $
 ** Stack and Call structure of Lua
 ** See Copyright Notice in lua.h
 */
@@ -571,6 +571,11 @@ LUA_API int lua_resume (lua_State *L, lua_State *from, int nargs) {
 }
 
 
+LUA_API int lua_isyieldable (lua_State *L) {
+  return (L->nny == 0);
+}
+
+
 LUA_API int lua_yieldk (lua_State *L, int nresults, int ctx, lua_CFunction k) {
   CallInfo *ci = L->ci;
   luai_userstateyield(L, nresults);