A coroutine can close itself

A call to close itself will close all its to-be-closed variables and
return to the resume that (re)started the coroutine.

lstate.c

@@ -326,6 +326,8 @@ LUA_API int lua_closethread (lua_State *L, lua_State *from) {
   lua_lock(L);
   L->nCcalls = (from) ? getCcalls(from) : 0;
   status = luaE_resetthread(L, L->status);
+  if (L == from)  /* closing itself? */
+    luaD_throwbaselevel(L, status);
   lua_unlock(L);
   return APIstatus(status);
 }