linked-list: Implement canonical-data.json

[kytrinyx]

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 Linked List exercise can be found in the ./exercises/linked-list/ directory.

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

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

The tracking issue has more context.

[stkent]

When working on this, it would be nice if simple-linked-list ended up being, ahem, simpler, than linked-list. I'm imagining that the functionality demanded by simple-linked-list should be a subset of the functionality demanded by linked-list, and that the tests for simple-linked-list should similarly be a subset of the tests for linked-list.