Issue 25/avoiding the missing generation of getters for the test build

[diseraluca]

Resolves #25.

Recently, the getters for the priority field in
uavcan::session_id::message::MessageSessionId were disabled from being
generated to avoid an unused warning, as they are, indeed, unused in the build
codebase.

Nonetheless, the getter for priority is actually used by one of the tests, such
that is needed, but only when testing.

The general way to do this, would be to add a #[cfg_attr(...)] for the skip
attribute to avoid generating it during testing.

Unfortunately, cfg* attributes are not expanded before proc_macro_attributes
expansion and the modular-bitfield does not manage them manually.

This is a general problem in the proc-macro environment, with an example being
taiki-e/pin-project#68.

The rust team is working on a solution to this, see, for example:

https://doc.rust-lang.org/beta/unstable-book/library-features/cfg-eval.html
https://doc.rust-lang.org/beta/unstable-book/language-features/macro-attributes-in-derive-output.html

And the related issues.

In particular, cfg_eval provides a way to ensure that the cfg attributes are
evaluated before the expansion of the following macros.

For this reason, the cfg_eval was activated, a #[cfg_eval] attribute was
added to MessageSessionId and a cfg_attr conditioned on test was added to
the priority field of the same structure.

As this is not yet stabilized, the crate now depends on nightly, and cannot be
compiled in stable.

It was possible to provide workarounds to avoid moving outside stable.
For example, by duplicating MessageSessionsId and providing a version with the
skip attribute and one without, conditioning the whole structure, instead of a
field attribute, on test.

Nonetheless, being nightly only is not considered a problem for this crate
usage and the "forward-correct" solution was thus preferred.

• A unused mut in the tests that escaped Correct the compilation warnings #10 was removed.