CI with RTIC as a dependency

[perlindgren]

The current CI is run internally on the RTIC repo, which does not cover edge cases of build.rs dependencies.

Possible solution:

• Make a sub-crate with proper dependency to RTIC.
• Include tests in the sub-crate.
• Integrate to CI.

[perlindgren]

This will be solved by : #623

[AfoHT]

This has been implemented for RTIC v1 and v0.5 in rtic-examples repo.

For current RTIC v2 examples lives in rtic/examples, in the repo but distinct.

Using rtic, rtic-monotonics, rtic-time, rtic-sync etc. with both direct path (relative) and some with crates.io dependencies.