Introduce `BlockHashProvider` interface

[chfast]

This is OOP at 100%.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.22%. Comparing base (8ed5293) to head (85e98fa). Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1059   +/-   ##
=======================================
  Coverage   94.21%   94.22%           
=======================================
  Files         158      158           
  Lines       17055    17065   +10     
=======================================
+ Hits        16069    16080   +11     
+ Misses        986      985    -1     

Flag	Coverage Δ
eof_execution_spec_tests	17.54% <62.50%> (+0.03%)	:arrow_up:
ethereum_tests	26.99% <91.66%> (+0.04%)	:arrow_up:
ethereum_tests_silkpre	19.12% <41.17%> (+0.02%)	:arrow_up:
execution_spec_tests	20.26% <87.50%> (+0.04%)	:arrow_up:
unittests	89.27% <100.00%> (+0.01%)	:arrow_up:

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

Files with missing lines	Coverage Δ
test/state/block.hpp	100.00% <ø> (ø)
test/state/host.cpp	100.00% <100.00%> (ø)
test/state/host.hpp	100.00% <100.00%> (ø)
test/state/state.cpp	99.59% <100.00%> (ø)
test/state/state.hpp	100.00% <ø> (ø)
test/state/state_view.hpp	100.00% <100.00%> (ø)
test/state/system_contracts.cpp	100.00% <100.00%> (ø)
test/state/test_state.cpp	100.00% <100.00%> (+2.17%)	:arrow_up:
test/state/test_state.hpp	92.30% <100.00%> (+3.41%)	:arrow_up: