Refactor GraphTraversal to avoid non-determinism (#4098)
### Description
The current design of the `Visit` trait makes it easy to introduce
non-determinism by having `map_children` return different results
depending on the order in which it's called.
For instance, if `map_children` tries to de-duplicate children (as is
the case in `ChunkContentVisit`):
```
Order 1:
1. (from parent Node1) map_children([NodeA, NodeB]) -> GraphTraversalControlFlow::Continue([NodeA, NodeB])
2. (from parent Node2) map_children([NodeB, NodeC]) -> GraphTraversalControlFlow::Continue([NodeC])
Order 2:
1. (from parent Node2) map_children([NodeB, NodeC]) -> GraphTraversalControlFlow::Continue([NodeB, NodeC])
2. (from parent Node1) map_children([NodeA, NodeB]) -> GraphTraversalControlFlow::Continue([NodeA])
```
These two calling orders will result in different generated graphs, as
the first order will forget about the edge Node2 -> NodeB, while the
second will forget about the edge Node1 -> NodeB.
Instead, this PR makes it so `map_children` is called *after* inserting
all nodes into the graph. This ensures that a different ordering can't
affect the final shape of the graph.
It also refactors the `GraphTraversal::visit` method and the `Visit`
trait to make it more consistent with graph terminology and (hopefully)
easier to understand.
### Testing Instructions
This solves an issue on the branch sokra/repro-for-alex.
