Implement display_hint in gdb pretty printers

[tromey]

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.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

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[carols10cents]

Thanks for the PR! We’ll periodically check in on it to make sure that @aturon or someone else from the team reviews it soon.

[carols10cents]

Actually, @aturon is on PTO this week, so let's try....

r? @michaelwoerister

[michaelwoerister]

@bors r+ rollup

Thanks @tromey! If anybody knows how to correctly use GDB's python API ... :)

[bors]

📌 Commit c3c1df5 has been approved by michaelwoerister