op-batcher: make shadow compressor more memory efficient

[geoknee]

WIP

#16537

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.24%. Comparing base (65117fa) to head (fc8e1f2).
Report is 21 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #16570       +/-   ##
============================================
+ Coverage    82.39%   96.24%   +13.85%     
============================================
  Files          158      106       -52     
  Lines         8817     4584     -4233     
============================================
- Hits          7265     4412     -2853     
+ Misses        1412      172     -1240     
+ Partials       140        0      -140     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	96.24% <ø> (ø)

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see 52 files with indirect coverage changes

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[geoknee]

Through local profiling (see #16572) I am seeing a 2x-3x improvement in memory usage per pending channel with this change.

[geoknee]

However, as is this change is causing safe head progression tests to fail, so that needs to be addressed!