[perf] Optimize -parent-child-transformer

[alexander-yakushev]

Computing a child-transformer in a single reduce achieves several benefits:

1. No need to accumulate intermediate child transformers into a vector.
2. No need to reverse the vector direction - calling -comp on the next child and the accumulator achieves that naturally.
3. For the very common case where there is just one child, the vector overhead is gone completely.

[alexander-yakushev]

@frenchy64 Yeah, that was my initial version, but then I decided to codegolf one line off. I've commited your suggestion.