First "complete" implementation of to-be-closed variables

Still missing:
- handling of memory errors when creating upvalue (must run closing
method all the same)
- interaction with coroutines

ldo.h

@@ -50,6 +50,7 @@
 /* type of protected functions, to be ran by 'runprotected' */
 typedef void (*Pfunc) (lua_State *L, void *ud);
 
+LUAI_FUNC void luaD_seterrorobj (lua_State *L, int errcode, StkId oldtop);
 LUAI_FUNC int luaD_protectedparser (lua_State *L, ZIO *z, const char *name,
                                                   const char *mode);
 LUAI_FUNC void luaD_hook (lua_State *L, int event, int line,