[Enhancement] Add optree integration to etils.etree

[XuehaiPan]

optree is a standalone package (like dm-tree) aimed to high-performance PyTree manipulation (like jax.tree_util). It offers similar APIs to jax.tree_util but better.

Some initial benchmark results:

Average Time Cost (↓)	OpTree (v0.9.0)	JAX XLA (v0.4.6)	PyTorch (v2.0.0)	TensorFlow Nest (v2.12.0)	DM-Tree (v0.1.8)
Tree Flatten	x1.00	2.33	22.05	1.38	1.12
Tree UnFlatten	x1.00	2.69	4.28	13.69	16.23
Tree Flatten with Path	x1.00	16.16	Not Supported	21.10	27.59
Tree Copy	x1.00	2.56	9.97	9.62	11.02
Tree Map	x1.00	2.56	9.58	9.16	10.62
Tree Map (nargs)	x1.00	2.89	Not Supported	74.26	31.33
Tree Map with Path	x1.00	7.23	Not Supported	40.78	19.66
Tree Map with Path (nargs)	x1.00	6.56	Not Supported	69.63	29.61

We have already seen some etils folks get involved with optree and jax.tree_util discussions. I wonder if etils maintainers have interest to add optree to etils.etree.

Ref:

• jax.tree_utils do not keep dict key order jax#4085
• [Feature Request] Preserve dict key order in the output of tree_unflatten, tree_map, and tree_map_with_path metaopt/optree#45
• feat: preserve dict key order in the output of tree_unflatten, tree_map, and tree_map_with_path metaopt/optree#46

[Conchylicultor]

Good idea. Let me try this