This creates distinct tutorial examples from complex feature examples
(more how-tos). Both sets are getting builder / derive versions (at
least the critical ones).
This ports our example testing over to [trycmd](https://docs.rs/) so
we can:
- More thoroughly test our examples
- Provide always-up-to-date example usage
The old way of testing automatically picked up examples. This new way
requires we have a `.md` file that uses the example in some way.
Notes:
- Moved overall example description to the `.md` file
- I added cross-linking between related examples
- `14_groups` had a redundant paragraph (twice talked about "one and
only one"
Because we gradually build the `ArgGroup` as we parse the YAML, we don't
use `ArgGroup::new`. Clap3 introduced an internal `id` in addition to
the public `name` and it appears that this custom initialization code
was not updated.
This shows the problem with publically exposing `impl Default`.
Choices:
- Remove `impl Default`
- Always valid
- Requires spreading invariants
- Callers can't implement code the same way we do
- Add `ArgGroup::name`
- Can be constructed in an invalid state
- Centralizes invariants
- A caller could implement code like the yaml logic
I decided to go with `ArgGroup::name`.
Fixes#2719
