I doubt list concatenation works like that...

To make sure not to execute an unexpended pipeline, use generics to
separate pre- and post- expansion pipelines
Add a pre-command callback

Merge errors.
Parse using the FromStr trait rather than ad-hoc parse methods

Breaking change: if a signal is terminates a subprocess, the last status
is no more set to 128 + signal

When Ion is started as a login shell, the `login` command expects the
shell to take ownership of the terminal. Else, the read command fails,
putting liner in an infinite loop. Bring back the set_unique_pid
function