DBC writing, merging, refactor, and other new features

[MerrimanInd]

Hey Marcel,

I forked your repo initially to add the ability to read in multiple DBCs, merge them, and write the output. Eventually I need a solution to also be able to create DBCs from scratch in Rust and export the file, so more compositional features. This has required a lot of restructuring and eventually I may have to break away from your data model to add better linking between attributes and their respective signals/messages.

Before I even open a PR, I see that in other issues you've not been interested in modifications to that degree. Would you be interested in looking at a significant rework of your repo or should I just break the fork and rerelease it as my own crate? I always prefer cooperation over multiple redundant crates in the ecosystem but it's your project to maintain!

Thanks,
-Xander