NACK. It's less lines, but less self-documenting. Semantic information as well as encapsulation is lost

Another argument is moving more logic out of the primitives, the same reason why @sipa didn't wanted primitives/block to depend on pow.o
But I don't have a strong opinion on this, one more nack or lack of interest for a while and I'm closing it.

@laanwj Would making a IsTxInFinal function in main be acceptable?

I think at that point actual documentation would be better than self-documentation.
Anyway, I'm closing. We can come back to this after isfinaltx has been moved to consensus.o