[Feature] Finish Code coverage by tests

This ticket is to track the various folders that are missing rust tests as 17/12/25:

* [ ] `meta/src`
* [ ] `src/compiler`
* [ ] `src/modules`
* [ ] `src/optimizer`
* [ ] `src/translate`
* [ ] `src/utils`

For reference: https://app.codecov.io/gh/amber-lang/amber

CodeCov don't detect Amber but we are covering a lot of features of the language but it is important to cover also the Rust part.