Deprecates the maktaba#system API in favor of a builder-pattern implementation in maktaba#syscall

Refactors the maktaba#system API into a builder-pattern API similar to maktaba#function that draws a clearer separation between "what" and "how". maktaba#system is still around for backwards compatibility, and delegates everything to maktaba#syscall.

The existing API is pretty haphazard with a bunch of arbitrary positional args all over the place. I've been wanting to support STDIN (#9) and also thinking about some kind of deferred execution (on CursorHold), and I didn't see a way to cram them into the existing API without creating something truly monstrous.

Feedback welcome.