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