state: Simplify code modification indicator in StateDiff

[chfast]

Use std::optional to indicate that an account's code has changed in the state diff.

[chfast]

For testing, I will rebase #961 on top of this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.29%. Comparing base (f9f328b) to head (3e16ad7).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1117   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files         159      159           
  Lines       17323    17325    +2     
=======================================
+ Hits        16335    16337    +2     
  Misses        988      988           

Flag	Coverage Δ
eof_execution_spec_tests	15.91% <92.30%> (+<0.01%)	⬆️
ethereum_tests	26.89% <76.92%> (+<0.01%)	⬆️
ethereum_tests_silkpre	18.98% <83.33%> (+0.01%)	⬆️
execution_spec_tests	21.10% <61.53%> (+<0.01%)	⬆️
unittests	89.03% <92.30%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
test/state/account.hpp	100.00% <ø> (ø)
test/state/host.cpp	100.00% <100.00%> (ø)
test/state/state.cpp	98.44% <100.00%> (ø)
test/state/state_diff.hpp	100.00% <ø> (ø)
test/state/test_state.cpp	100.00% <100.00%> (ø)

[chfast]

I test-rebased eip-7702 on top of this and it looks smooth and all tests passed.