Optimize flatten/unflatten

[cgarciae]

Changes

• Adds _pytree__node_fields static field that is calculated for all Pytrees after __init__.
• Uses _pytree__node_fields to avoid sorting during tree_flatten.
• Uses flatten_func from register_pytree_with_keys to speedup flattening when keys are not needed.

[codecov]

Codecov Report

Merging #12 (279581c) into main (a4bc7a8) will decrease coverage by 1.14%.
The diff coverage is 92.45%.

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
- Coverage   97.45%   96.32%   -1.14%     
==========================================
  Files           2        3       +1     
  Lines         118      136      +18     
==========================================
+ Hits          115      131      +16     
- Misses          3        5       +2     

Impacted Files	Coverage Δ
simple_pytree/dataclass.py	90.00% <90.00%> (ø)
simple_pytree/pytree.py	97.32% <93.10%> (-0.07%)	⬇️
simple_pytree/__init__.py	100.00% <100.00%> (ø)

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