debug flags "print_attributes" and "print_attrs"
"print_attributes" is declared in Pretty and Expr (should be exported from Pretty, really) and is used in pretty-printing of terms and expressions "print_attrs" is declared in Call_provers and Itp_server.
Do we need two different debug flags or should we merge them? If we merge, I'd recommend using "print_attributes". If we do not, I'd recommend renaming "print_attrs" and write a clear explanation of what it does.