Create an extras crate with a bunch of more or less useful extra features

[Kile-Asmussen]

Example extra thingies:

• Tokio utility functions (feature gated)
  • When processing futures concurrently, collect all errors into child reports
  • Maybe something with the most popular retry crate?
• If some of the extra attachment/context handlers from Add more default attachment handlers #106 are found to not be useful enough to go in the main crate, these can go here
• Serde integration (feature gated)
  • Report formatter hook that makes JSON?
  • Utility functions for arbitrary serialization formats
  • Some sort of type map that recognizes which attachments/contexts implement Serialize and defaults to strings otherwise
  • Display = pretty print, Debug = terse?
  • Appendix/InlineWithHeader support?
• More report formatters
  • Attachment filtering features
  • Child report filtering features
• More report creation hooks
  • Timestamp attachments and such (SystemTime)
• A library of utility functions for traversing Reports as a data structure:
  • searching for attachments/contexts of specific types/satisfying predicates
  • enumerating child reports in depth-first or breadth-first manners
  • exposing the enumeration of attachments according to priority

[TethysSvensson]

This all sounds good to me. I don't have a strong opinion on whether this should live in the same repository, in a different repository in the rootcause-rs organisation or externally. Do you have any preferences here?

If it's going to live in a different repository, then it would probably make sense to close this tracking issue and open one on the other repository once it's been created.

[Kile-Asmussen]

I think putting it in the same repo is fairly sane. Like an extra, separate examples crate.