Forwarding I/O activity
=======================

   An "indirectbuf" is one that forwards all of its I/O requests to
another streambuf.

   An `indirectbuf' can be used to implement Common Lisp
synonym-streams and two-way-streams:

     class synonymbuf : public indirectbuf {
        Symbol *sym;
        synonymbuf(Symbol *s) { sym = s; }
        virtual streambuf *lookup_stream(int mode) {
            return coerce_to_streambuf(lookup_value(sym)); }
     };