This refactors the engine so all of the APIs on it are instance
methods on the type instead of raw methods that float around and use
data from a global engine.
A mechcanical change as we remove the global `E` and then make
anything that interacted with that in pkg/engine to be an instance
method and the dealing with the fallout.
Instead of talking directly to stdout or stderr via methods on fmt,
indirect through an Engine type (presently a global, but soon to
change) to allow control of where the streams actually end up.
Prevously, we would throw raw args arrays across the interface and the
engine would do some additional parsing. Clean this up so we don't do
that and all the parsing stays in `lumi`
