`multicall` allows you to have one binary expose itself as multiple
programs, like busybox does. This also works well for user clap for
parsing REPLs.
Fixes#2861
`-h` (short help) still shows the same.
This gates it behind an `unstable-v4` feature flag to avoid disrupting users who set the help without knowing where all it shows up (particularly derive users where `ArgEnum` is automatically extracting the help).
Fixes#3312
These were broken in #17 and #38 when renaming `hidden` to `hide`. These
weren't caught because we test with `--all-targets` which is mutually
exclusive with `--doc`.
In #27, we removed some default features. When doing so, some places
weren't updated but `doc` feature covered it ... except it was only
partially updated. This makes sure we test all the features.
