This commit brings the cli `hpk` binary back into the `hpk` crate and
places it behind a feature flag, enabling development to take place in a
single repository.
entries fully multithreaded including compressing into the archive, as
well as passing messages through an mpsc::channel() interface for use in
a background thread in various frontends. Much better suited to task
than previous `create_package` function. Untested.
