Rename keccak library -> ethash library

[AndreaLanfranchi]

Removed extern C stuff
Propedeutic to full ethash implementation

[codecov]

Codecov Report

Merging #229 (c9062eb) into master (f536656) will increase coverage by 0.25%.
The diff coverage is 98.14%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   78.66%   78.91%   +0.25%     
==========================================
  Files          74       74              
  Lines        6561     6521      -40     
==========================================
- Hits         5161     5146      -15     
+ Misses       1400     1375      -25     

Impacted Files	Coverage Δ
ethash/ethash/keccak.cpp	99.29% <98.13%> (ø)
ethash/ethash/keccak.hpp	100.00% <100.00%> (ø)

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