Tidy up the codebase

[bartkl]

Description
Let's tidy up the code base by providing somewhat more information and structure.

Acceptance criteria

• Project has no lingering or unused files
• The modules as partitioned make sense
• Public and private functions and clearly demarcated
• Public functions have Malli schemas and documentation associated with them; private functions only where this feels useful
• Copyright headers include 2023 (starting 2022 I believe)

Out of scope

• Improved error messages (will follow thanks for Malli)
• Unit tests
• Difficult refactoring: stick with easy move-arounds