Use div_ceil in tree_hash impl

[macladson]

This fixes the warning that clippy was raising about manually re-implementing div_ceil.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.07%. Comparing base (dd6ac18) to head (12b0ac1).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tree_hash.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
- Coverage   58.62%   58.07%   -0.55%     
==========================================
  Files           8        8              
  Lines         290      291       +1     
==========================================
- Hits          170      169       -1     
- Misses        120      122       +2     

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