Task dependencies

[bitprophet]

🚨 TODO: copy in stuff from the Fabric ticket 🚨

Might be nice to have the API for "I depend on these other tasks running first" be *args in @task, e.g. @task(other1, other2, kwargs=here). Make-like, simple, *args aren't sensible otherwise (e.g. to call the existing things intended to be kwargs). Props to @mattrobenolt for the idea.

[bitprophet]

Pre-tasks are now implemented using an Executor class, which should also form the basis for post-tasks/parameterization/parallelization/etc.

Still using the *args shortcut as per above & as per Matt's earlier PR.

[bitprophet]

Still need to implement the now-in-doc deduping of tasks too.

[bitprophet]

Deduping is in! For now, used a dedupe bool kwarg to Executor.execute (vs using a distinct subclass) but I'm sure this will adapt as we go, especially re #45.

That set of changes also (as with Matt's earlier PR) implements a (basic, not preserving any metadata) Task.__call__.

[bitprophet]

Implemented --no-dedupe in basic form, calling this done.