Update .atta serialization

[brenocq]

Feature description
The current .atta stores a JSON-like format in binary format, which is great for having a small memory footprint, but horrible for the user experience (lack of version control and not possible to edit values by hand). The format should be changed to plain text so that changes to .atta with git can be tracked.

Tasks

• Define new plain-text format
• Implement plain-text format
• Migrate some of the old examples to the new format