Implement display_hint in gdb pretty printers
A few pretty-printers were returning a quoted string from their
to_string method. It's preferable in gdb to return a lazy string and to
let gdb handle the display by having a "display_hint" method that
returns "string" -- it lets gdb settings (like "set print ...") work, it
handles corrupted strings a bit better, and it passes the information
along to IDEs.
fab96c4b12