Add missing translator tests for C++, Nim and Rust

[Mingun]

Do not merge yet, it is not finished. Not a draft because I want to run CI checks

I noticed that TranslatorSpec.scala does not test some languages, which seems to be oversight for Nim, and unfinished work for Rust.

Nim compiler seems to be production ready, so tests needs to be added.

Rust compiler is not finished yet, and tests should make finishing it easy.

[pombredanne]

Thanks for the work there. Rust would be awesome!

[generalmimon]

@pombredanne:

Rust would be awesome!

There is already quite decent support for Rust on unreleased master branches, as explained at https://app.gitter.im/#/room/!gxFYJoZuKcRouBfhlb:gitter.im/$U2qXwDnOd6HKOMzdHpcXehPmfg95pxgP4VA2D0CTtMk. So if you don't mind building the Kaitai Struct compiler from source (which is actually easy, as you can see at https://doc.kaitai.io/serialization.html#_building_the_compiler_from_source - just make sure to check out the master branch, not serialization as it says), you can already try it out now before a new version of Kaitai Struct is released. If you do, please give us feedback in kaitai-io/kaitai_struct#22 or open issues in https://github.com/kaitai-io/kaitai_struct/issues for bugs/suggestions/questions.