The generated code with `()` doesn't compile in most of the cases. To signal
the developer there's something to do, fill in `todo!()`.
Because the file *missing_fields.rs* contains the string `todo!()` it needs
an exception for the test *check_todo*.
Moving tests to `rust-analyzer` crate allows removing walkdir dependency
from `xtask`. It does seem more reasonable to keep tidy tests outside of
the "build system" and closer to other integration tests.
* Keep codegen adjacent to the relevant crates.
* Remove codgen deps from xtask, speeding-up from-source installation.
This regresses the release process a bit, as it now needs to run the
tests (and, by extension, compile the code).
