Create separate crate for helper types and external dependencies

[scootermon]

The main issue I want to address is that the generated code currently contains the CanError enum. I have multiple DBC files for different can buses that I would like to abstract in a single crate, but having multiple equal but distinct CanError types makes this really annoying.

To solve this I think it would be good to introduce a separate runtime crate. This crate can then hold common types and also re-export the additional dependencies used by the extra code.
The latter can be done through a hidden module like:

#[doc(hidden)]
mod __priv {
  pub use bitvec;
  // [...]
}

I'm happy to work on an initial implementation if we're happy with this approach.

[killercup]

What do you think of #35? I'd be happy to merge some big refactors if you can motivate them :)

[scootermon]

#35 is exactly the use case I wanted to use this crate for initially. I have a repository of dbc files and I wanted to just iterate over all of them, generate a crate for each one, and publish them to an internal registry.