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

The tracking issue has more context.

[NobbZ]

Hasn't this got deprecated in favor of all-your-base alongside of binary, trinary, and octal?

[kytrinyx]

It should be, but it apparently doesn't have a .deprecated file. I'll open an issue.

[kytrinyx]

There's already an open issue: #279