Migrate benchmarks to the JSON State Test format

[miles170]

Closes #511.

I haven't checked gas used in every benchmark loop iteration because I want to make sure current changes won't break anything.
After that, I'll add a check for gas used.

[codecov]

Codecov Report

Merging #513 (ec9e5ae) into master (13a7873) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files          54       54           
  Lines        5341     5344    +3     
=======================================
+ Hits         5291     5294    +3     
  Misses         50       50           

Flag	Coverage Δ
blockchaintests	77.78% <ø> (ø)
statetests	7.55% <100.00%> (+0.05%)	⬆️
unittests	97.18% <0.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
test/state/state.hpp	80.00% <ø> (ø)
test/statetest/statetest.hpp	100.00% <ø> (ø)
test/statetest/statetest_loader.cpp	81.35% <100.00%> (+0.48%)	⬆️

[miles170]

Should I add more test cases to GeneralStateTests(https://github.com/ethereum/tests)?

[chfast]

Should I add more test cases to GeneralStateTests(https://github.com/ethereum/tests)?

If you are asking in context of code coverage, then this is not needed. We don't run the tests yet. But on the other hand it is always welcomed to add new tests if you have any ideas.