paasio: Implement canonical-data.json

We want to have a standard set of test inputs and outputs for each exercise to
make it easier to port them to new languages, as well as to help keep the
exercises in sync.

The Paasio exercise can be found in the [./exercises/paasio/][exercise-dir] directory.

The step-by-step instructions for how to compile a canonical-data.json file is
described in [this section of the contributing guide][contributing].

See http://exercism.io/contribute/canonical-data/paasio for the up-to-date list of
language tracks that have an implementation of the Paasio exercise.

The [tracking issue][tracking-issue] has more context.

[contributing]: https://github.com/exercism/docs/blob/master/you-can-help/improve-exercise-metadata.md#extracting-canonical-test-data
[tracking-issue]: https://github.com/exercism/x-common/issues/552
[exercise-dir]: https://github.com/exercism/x-common/tree/master/exercises/paasio