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%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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.