Bebop "message" and "union" definition benchmarks

Hello,

It would be nice to have Bebop message and union definition(s) benchmarked as well. In practice, I find myself relying on message and union more often than bare struct and I suspect that would be the case for most non-trivial applications as well. Since these are less efficient than struct, it would be interesting to see how they fare.

[200sc]

I did some work on this, and also added some (unrealistic) optimizations. I don't know if this is something that should be merged back into here, but it's on this branch for perusal: https://github.com/200sc/go_serialization_benchmarks/tree/bebop-2