[C++] Fix issues with length computation.

I had incorrectly assumed that the `Flyweight::computeLength` method
took _encoded lengths_ of groups etc., but actually it takes a
complicated structure of group counts and variable lengths. As it was
hard to build this list, I've opted for a simpler approach: do the
length calculation within the generated DTO message and its groups.

In this commit, I've also added some convenience methods for converting
between DTOs and "byte arrays".