Separate traversal from representation with generic fold

[RoyalPineapple]

Summary

Separates the tree traversal from the output representation by making mapNodesToHierarchy generic.

Before:  walk → AccessibilityNode tree → mapNodesToHierarchy() → [AccessibilityHierarchy]

After:   walk → AccessibilityNode tree → foldNodes<Node>()     → [Node]
                                          ├── makeElement(element, index, source) → Node
                                          └── makeContainer(container, children, source) → Node

parseAccessibilityHierarchy(in:) becomes a two-line wrapper that calls the fold with closures producing AccessibilityHierarchy. No behavioral change for existing callers.

Usage

parser.parseAccessibilityHierarchy(
    in: view,
    makeElement: { element, traversalIndex, source in
        MyNode.leaf(element, view: source)
    },
    makeContainer: { container, children, source in
        MyNode.group(container, children: children, view: source)
    }
)

source is the live NSObject that produced the element or container.

[soroushsq]

This is purely additive in terms of APIs, right? The old APIs are composed out of this new one?

[soroushsq]

Can this one become generic over (String, Source?, Container?). I guess it's not a generic reduce, so it has to return an array of some kind or other?

[RoyalPineapple]

it does need to be an outer array but the generic can be a tuple like that, updated the test to use
typealias FoldNode = (label: String, source: NSObject, container: AccessibilityContainer?)