Digraph, trigraphs and alternate operators

Moved digraph and trigraph-handling out from cpp_tokenizer. This will be
done from pp_tokenizer now. The alternate operators are just macros,
defined in iso646.h. See
http://gcc.gnu.org/onlinedocs/cpp/C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators