Meta-issue / design principle: minimize implicit arguments, global arguments, and singletons

To facilitate reasoning and remove sources of hangs and bugs, we need to remove all global state and singletons. This means we should refactor the `@local` and `@vectorize` decorators as well, since these need to implicitly access an IPython.parallel `Client`.

We can revisit relaxing this in the future, but for now, this is the standing policy.