fix: Reduce use of spread operator

Per @Qrtn's suggestion in:
#73 (comment)

Also makes the large array test use a
more realisitic result shape, with objects
with various contents.

src/traverseTree.test.ts

@@ -359,9 +359,15 @@ describe('traverseTree', () => {
     ])
   })
 
+  const leaf = (index: number) => ({
+    hello: 'world',
+    index
+  })
+  const array = Array.from({ length: 1_000_000 }, (_, i) => leaf(i))
+
   test('root node is an array with 1 million rows', () => {
     const visitor = jest.fn().mockImplementation(state => state)
-    traverseTree(Array.from(Array(1000000).keys()), visitor, null)
-    expect(visitor).toHaveBeenCalledTimes(1000001)
+    traverseTree(array, visitor, null)
+    expect(visitor).toHaveBeenCalledTimes(3000001)
   })
 })

src/traverseTree.ts

@@ -32,7 +32,7 @@ export function traverseTree<State>(
     state: State
   }
 
-  let stack: StackItem[] = [
+  const stack: StackItem[] = [
     {
       path: [],
       type: typeOf(input),
@@ -56,7 +56,9 @@ export function traverseTree<State>(
         state: newState
       })
     )
-    stack = [...stack, ...children.reverse()]
+    for (let i = children.length - 1; i >= 0; --i) {
+      stack.push(children[i])
+    }
   }
 }