This adds an `untested` feature to any functions which have not yet been
tested live against a YubiKey device (which is presently pretty much
everything).
This sets a clear expectation of what is presently supported, and
additionally documents the status in the README (and a series of GitHub
issues).
Adds a `cargo build --all-features` to GitHub Actions' `test` step in
order to make sure that `untested` functionality still compiles.
Tony Arcieri
a23af7dc31