ATD modules (work in progress)

[mjambon]

This provides an import construct that serves as declaring the existence of a module. A module m is derived from an ATD file m.atd or some implementation that exposes a compatible interface. A type t provided by module m is referred to as m.t.

Implements #265

PR checklist

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

[mjambon]

This attempt to support dedicated syntax for imports failed due to the complexity of the atdgen implementation. The problem is that atdgen generates code that doesn't read or write via a generic JSON AST but instead reads and writes JSON directly to/from bytes. This makes things unmanageable. I didn't have any problems supporting import for the other target languages (Python, TypeScript, ...) which all go through a JSON AST of sorts.

Here's what I suggest:

1. Salvage what's salvageable from this PR i.e. add support for import for all languages except OCaml/atdgen.
2. Implement a new, simpler OCaml code generator named atdml that would support imports.