refactor(durable-storage): hashing the MerkleLayer cannot fail

[vapourismo]

What

Remove the fallibility from hashing operations in the Merkle layer.

Why

These operations can't fail.

Manually Testing

make all

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Explain changes to regression test captures when applicable.
• Write commit messages in agreement with our guidelines.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.76%. Comparing base (be8ca56) to head (86f9324).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #956      +/-   ##
==========================================
- Coverage   89.82%   89.76%   -0.06%     
==========================================
  Files         123      123              
  Lines       26469    26455      -14     
  Branches    26469    26455      -14     
==========================================
- Hits        23775    23747      -28     
- Misses       2082     2097      +15     
+ Partials      612      611       -1     

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[github-actions]

Benchmark results for revision c16766d:

Metric	Duration	TPS
Mean	1.516251168s	26.381
Worst	1.528160921s	26.175
Best	1.508351464s	26.519
Standard Deviation	±5.803951ms	±0.101

Full results

Run	Transfers	Duration	TPS
1	40	1.526476497s	26.204
2	40	1.518827593s	26.336
3	40	1.518750068s	26.337
4	40	1.517364492s	26.361
5	40	1.50975476s	26.494
6	40	1.515971579s	26.386
7	40	1.514740876s	26.407
8	40	1.508523086s	26.516
9	40	1.5171165s	26.366
10	40	1.518872417s	26.335
11	40	1.511843973s	26.458
12	40	1.528160921s	26.175
13	40	1.51133967s	26.467
14	40	1.526979866s	26.195
15	40	1.515012876s	26.402
16	40	1.509778436s	26.494
17	40	1.515512651s	26.394
18	40	1.520059051s	26.315
19	40	1.511586578s	26.462
20	40	1.508351464s	26.519

Compare the results above with those for the default branch.