ATDD : ATDgen backend to generate `dlang` code

[elrandar]

ATDD

This PR adds the atdd backend, generating dlang serializers and deserializers from ATD definitions. Supported features are:

• optionals
• default values
• sumtypes
• wrapping and unwrapping with annotations
• specifying dlang imports with top level annotations eg: <dlang import="std.stdint : uint32_t">

Also adds unit tests for testing and checking compilation of generated code. To execute those the ldc compiler is needed, more information in readme.md.

PR checklist

• [x] New code has tests to catch future regressions
• [ ] Documentation is up-to-date
• [x] CHANGES.md is up-to-date

[Khady]

git history can probably be squashed?

[Khady]

I forgot to switch to squash when I merged the PR, sorry about that. If we want to cleanup the history the master branch could be forced pushed

[mjambon]

A heads-up: I'm planning to release atd (all opam packages) tomorrow or early next week. This will include atdd.