Towards "to closed" local variables

Start of the implementation of "scoped variables" or "to be closed"
variables, local variables whose '__close' (or themselves) are called
when they go out of scope. This commit implements the syntax, the
opcode, and the creation of the corresponding upvalue, but it still
does not call the finalizations when the variable goes out of scope
(the most important part).

Currently, the syntax is 'local scoped name = exp', but that will
probably change.

lgc.c

@@ -293,7 +293,8 @@ static void reallymarkobject (global_State *g, GCObject *o) {
       gray2black(o);
       break;
     }
-    case LUA_TUPVAL: {
+    case LUA_TUPVAL:
+    case LUA_TUPVALTBC: {
       UpVal *uv = gco2upv(o);
       if (!upisopen(uv))  /* open upvalues are kept gray */
         gray2black(o);
@@ -760,6 +761,7 @@ static void freeobj (lua_State *L, GCObject *o) {
       luaF_freeproto(L, gco2p(o));
       break;
     case LUA_TUPVAL:
+    case LUA_TUPVALTBC:
       freeupval(L, gco2upv(o));
       break;
     case LUA_TLCL: