Ensure top-level can be used in `no_std` scenarios

[rvolosatovs]

We want to add some shared structs to this crate and we want to make them importable with no_std.
In order to achieve that, we can move firmware and launch-specific structs out of top-level to respective modules. The only thing left is then Version - that we can either guard with a feature flag along other module imports or move to a new module.
cc @haraldh

[rvolosatovs]

Since types would need to be moved for this, which would break semantic versioning guarantees if such change would be made after the release, setting milestone to v0.2.0