Define which functions, types, ... are the interface

[paulo-ferraz-oliveira]

It's important to get it right, for when exporting with ex_doc.

[paulo-ferraz-oliveira]

At the same time, review what we have currently, as to trim down to bare essentials, paying special attention to ignore_xref stuff.

[paulo-ferraz-oliveira]

Maybe restore xref for maintainability (?)

[paulo-ferraz-oliveira]

We need to:

• check what we want opaque
• how are types defined (are they complete?)
• how are functions spec.ed (are they complete?)