refactor(trie): extract save_account_branches closure to method4

[Himess]

Closes #566 (4/5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.39%. Comparing base (01da650) to head (c3d3828).

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/optimism/trie/src/backfill.rs	90.77% <100.00%> (+0.83%)	⬆️

... and 10 files with indirect coverage changes

Flag	Coverage Δ
e2e	35.79% <100.00%> (+<0.01%)	⬆️
unit	26.90% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	53.14% <ø> (ø)
op historical proof	90.01% <100.00%> (+0.04%)	⬆️

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