feat: any `Snafu` errors that are used in publicly available traits need some method for creating that error.

[ramfox]

Eg, in iroh-base, we have a trait Ticket, the method Ticket::from_bytes can return an iroh-base::ticket::ParseError, but there is no clean way to create a ParseError, since you would need to import other crates to fulfill the backtrace and span_tree fields inside the ParseError variant.