Add more default attachment handlers

[Kile-Asmussen]

This PR adds two probably quite useful attachment handlers:

• Hidden for entirely hidden attachments
• Opaque for attachments that are unimportant

It changes the behavior of Debug:

• The Debug handler no longer prefers using its display formatter. Formatting a report as {:?} with context/attachments using the Debug handler now prints the fmt::Debug text.
• A new handler RedactDebug has been added which prefers its display formatter, i.e. redacting the internal information, even when a report is formatted as {:?}.

It also introduces the concept of an attachment handler combinator, a simple wrapper type for another handler that adds something extra:

• Rendering with inline headers
• Rendering as appendixes
• Changing priority
• Using debug formatting for display
• (Technically Opaque is a combinator, but will probably not be used as one very often)

Whether any of these are useful enough to belong in the main library is up for debate.

[mkatychev]

What are your thoughts on naming this ForceDebug?

[mkatychev]

Thank you so much for adding this, it looks great!

Through my usage of .attach_custom::<Any, _>(some_t) this past week, it feels much less ergonomic to type out than something like .attach_opaque(some_t) especially if one is chaining multiple .attach_customs at once.

Do you have any thoughts on a ReportExt that would include methods for handlers included with error stack?

• attach_any
• attach_opaque
• attach_hidden
• attach_with_header
• attach_debug