Node trait as reusable visitor

[joseph-isaacs]

This is an idea of how to reuse the visitor for other tree structures, we need to find a place to pub the node trait, other than in vortex-expr.

see vortex-expr/src/transform for more complex examples.

This needs a code structure change.

[gatesn]

I don't think these children functions are quite right, allocating and returning vecs all over the place.

I'll have a think

[gatesn]

Going to kill this as stale