eof: Return constant hash of EXTCODEHASH of EOF

[chfast]

Replace runtime evaluated keccak256("EF00") with a constant returned for an account with EOF code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.21%. Comparing base (2cd8bf7) to head (02b6951).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1035   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files         153      153           
  Lines       15927    15933    +6     
=======================================
+ Hits        15005    15011    +6     
  Misses        922      922           

Flag	Coverage Δ
eof_execution_spec_tests	17.53% <75.00%> (+0.01%)	⬆️
ethereum_tests	27.34% <66.66%> (+<0.01%)	⬆️
ethereum_tests_silkpre	19.15% <45.45%> (+0.01%)	⬆️
execution_spec_tests	20.45% <41.66%> (+0.01%)	⬆️
unittests	89.00% <91.66%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
lib/evmone/eof.cpp	85.68% <100.00%> (ø)
lib/evmone/eof.hpp	100.00% <ø> (ø)
test/state/host.cpp	100.00% <100.00%> (ø)
test/unittests/eof_test.cpp	94.00% <100.00%> (+0.25%)	⬆️