ENH: changed return value to bool for Ostream::write(const token&)

- the return value signals if this method handled this particular type
  of token. This minor change allows this method to be used as a succinct
  prefilter an output token stream. It also provides better encapsulation
  of what the particular output stream handles.

  Eg,

      bool ok = os.write(tok);
      if (!ok)   // or   if (!ok && os.good())
      {
          os << tok;
      }

  instead of

      if (tok.type() == typeA || tok.type() == typeB || ...)
      {
          os.write(tok);
      }
      else
      {
          os << tok;
      }