chore: cleanup host blockhash fn

[rakita]

No description provided.

[github-actions]

Valgrind Results:

==3819== Cachegrind, a cache and branch-prediction profiler
==3819== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==3819== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==3819== Command: target/release/snailtracer
==3819== 
--3819-- warning: L3 cache found, using its data for the LL simulation.
Running snailtracer example!
elapsed: 1.783256839s
==3819== 
==3819== I   refs:      429,371,749
==3819== I1  misses:          3,093
==3819== LLi misses:          2,789
==3819== I1  miss rate:        0.00%
==3819== LLi miss rate:        0.00%
==3819== 
==3819== D   refs:      206,205,179  (133,650,671 rd   + 72,554,508 wr)
==3819== D1  misses:        344,815  (    204,754 rd   +    140,061 wr)
==3819== LLd misses:        137,632  (      4,276 rd   +    133,356 wr)
==3819== D1  miss rate:         0.2% (        0.2%     +        0.2%  )
==3819== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==3819== 
==3819== LL refs:           347,908  (    207,847 rd   +    140,061 wr)
==3819== LL misses:         140,421  (      7,065 rd   +    133,356 wr)
==3819== LL miss rate:          0.0% (        0.0%     +        0.2%  )

[onbjerg]

lgtm

[DaniPopes]

technically this breaks blockhash when block number is > usize::MAX because diff will always saturate to 0.