use cbuilder for seq type generation

[metagn]

addSimpleStruct is just so the compiler doesn't use so much extra computation on analyzing the typ parameter for addStruct, which doesn't change anything for seq types. We could probably still get away with using addStruct instead, or making addStruct accept nil as the typ argument but this would be even more computation.

There were a lot of hidden issues with addStruct being a template & template argument substitution, so most of the behavior is moved into startStruct/finishStruct procs.

This is turning out to be a lot of code for just a couple of changed lines, we might have to split cbuilder into multiple modules.