Add additional mechanisms to introspect state witnesses

[nagisa]

In particular this code is what I had used to inspect the contents of the state witnesses and the contribution of trie's contents to the state witness size.

This may or may not be useful, so am submitting this as a PR.

That said, at this point if it was possible to instead add support for witnesses to Debug UI, that would be much, much better.

[codecov]

Codecov Report

Attention: Patch coverage is 1.96078% with 100 lines in your changes missing coverage. Please review.

Project coverage is 71.58%. Comparing base (6b76ee6) to head (a679d95).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
tools/state-viewer/src/latest_witnesses.rs	0.00%	100 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #12177    +/-   ##
========================================
  Coverage   71.57%   71.58%            
========================================
  Files         821      824     +3     
  Lines      165303   165445   +142     
  Branches   165303   165445   +142     
========================================
+ Hits       118314   118429   +115     
- Misses      41852    41885    +33     
+ Partials     5137     5131     -6     

Flag	Coverage Δ
backward-compatibility	0.17% <0.00%> (-0.01%)	⬇️
db-migration	0.17% <0.00%> (-0.01%)	⬇️
genesis-check	1.26% <0.00%> (-0.01%)	⬇️
integration-tests	38.69% <1.96%> (+0.03%)	⬆️
linux	71.36% <1.96%> (-0.01%)	⬇️
linux-nightly	71.14% <1.96%> (-0.01%)	⬇️
macos	54.08% <1.96%> (+0.31%)	⬆️
pytests	1.52% <0.00%> (-0.01%)	⬇️
sanity-checks	1.32% <0.00%> (-0.01%)	⬇️
unittests	65.30% <1.96%> (+<0.01%)	⬆️
upgradability	0.21% <0.00%> (-0.01%)	⬇️

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