Avoid creating deep recursive calls with Command::and

[charypar]

Previous implementation of Command::and would create a cons-list shaped structure when folding over a list of commands with .and. This change makes it effectively equivalent to creating the folded list with ::all by wrapping the other command in .and to be hosted on selfs pair of channels, and spawning it in self.

As a side effect, this also adds a spawn API on a mutable command instance. Being pretty much equivalent with Command::new in what it can do, I think this can be public, but I could be talked out of that :).

There are potentially other similar optimisations in other places which wrap existing commands in new commands rather than merging them.