fix: U256 block_hash construction

[enitrat]

Closes #914

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.82%. Comparing base (5e34b74) to head (09a618d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #915      +/-   ##
==========================================
- Coverage   79.35%   77.82%   -1.54%     
==========================================
  Files          64       64              
  Lines       16699    16701       +2     
==========================================
- Hits        13252    12997     -255     
- Misses       3447     3704     +257     

Flag	Coverage Δ
ci	77.82% <100.00%> (?)
nightly	?

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