state: Support block hashes

[rodiazet]

Implement loading JSON block_number => block_hash map and expose it to Host::get_block_hash() via BlockInfo.

[codecov]

Codecov Report

Merging #694 (a536c7d) into master (6608cf3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
+ Coverage   97.51%   97.53%   +0.01%     
==========================================
  Files          86       86              
  Lines        8223     8249      +26     
==========================================
+ Hits         8019     8046      +27     
+ Misses        204      203       -1     

Flag	Coverage Δ
blockchaintests	63.19% <ø> (ø)
statetests	74.20% <57.14%> (-0.07%)	⬇️
statetests-silkpre	23.20% <21.05%> (-0.02%)	⬇️
unittests	95.30% <100.00%> (+0.19%)	⬆️

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Files Changed	Coverage Δ
test/state/state.hpp	100.00% <ø> (ø)
test/state/host.cpp	92.78% <100.00%> (+0.63%)	⬆️
test/statetest/statetest_loader.cpp	90.33% <100.00%> (+0.16%)	⬆️
test/unittests/state_transition_block_test.cpp	100.00% <100.00%> (ø)
...est/unittests/statetest_loader_block_info_test.cpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes