Disallow cargo from downloading dev-dependencies when they are not needed.

[diseraluca]

Currently, cargo build downloads all dependencies independently of if they are needed.

What this means for us, is that it tries to download and compile the dev-dependencies, which require std and are used only for tests and examples, even when building the library for embedded devices, where std is not present, failing the compilation.

Some possible solutions to this might be moving the example and tests in another crate, but this might not be possible as some tests may require access to private parts of the code and by rust convention should live in the module where the code is declared, moving to nightly where a solution to this problem is provided or hiding things behind a feature.