accumulate: 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 Accumulate exercise can be found in the ./exercises/accumulate/ 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/accumulate for the up-to-date list of
language tracks that have an implementation of the Accumulate exercise.

The tracking issue has more context.

[kytrinyx]

Accumulate should be folded into list-ops and deprecated.