Before #2005, `Clap` was a special trait that derived all clap traits it
detected were relevant (including an enum getting both `ArgEnum`,
`Clap`, and `Subcommand`). Now, we have elevated `Clap`, `Args`,
`Subcommand`, and `ArgEnum` to be user facing but the name `Clap` isn't
very descriptive.
This also helps further clarify the relationships so a crate providing
an item to be `#[clap(flatten)]` or `#[clap(subcommand)]` is more likely
to choose the needed trait to derive.
Also, my proposed fix fo #2785 includes making `App` attributes almost
exclusively for `Clap`. Clarifying the names/roles will help
communicate this.
For prior discussion, see #2583
Before, validating UTF-8 was all-or-nothing and would cause a `panic` if
someone used the right API with non-UTF-8 input.
Now, all arguments are validated for UTF-8, unless opted-out. This
ensures a non-panicing path forward at the cost of people using the
builder API that previously did `value_of_os` need to now set this flag.
Fixes#751
