Refactor some code

[l4l]

No semantic should be made there, if found it's an error.

Some other hints to improve the code:

• split up the god-types (i.e. the ones with >5 fields), in particular testing would be much easier;
• there's a lot of duplicated code, making it polymorphic might significantly reduce codebase size;
• use derive_builder for builders;
• use either anyhow with thiserror, or even better use snafu instead for error handling.