Tidy new support for cleanups

encode most things in Syms, do only as much work as necessary
(e.g. pending cleanups), don't track scopes in a large
structure (instead encode the scopes with cleanups directly
in the cleanups tree).  Removes ca. 120 lines of code.

tccpp.c

@@ -1366,7 +1366,6 @@ ST_FUNC Sym *label_push(Sym **ptop, int v, int flags)
     Sym *s, **ps;
     s = sym_push2(ptop, v, 0, 0);
     s->r = flags;
-    s->scope = scope_tracker;
     ps = &table_ident[v - TOK_IDENT]->sym_label;
     if (ptop == &global_label_stack) {
         /* modify the top most local identifier, so that