Fix Bitcoin public key generation

[alejandro-isaza]

Fix #18

[codecov-io]

Codecov Report

Merging #19 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   84.52%   84.58%   +0.06%     
==========================================
  Files          47       47              
  Lines        1783     1790       +7     
==========================================
+ Hits         1507     1514       +7     
  Misses        276      276

Impacted Files	Coverage Δ
Tests/CryptoTests.swift	100% <100%> (ø)	⬆️
Sources/Crypto.m	91.22% <100%> (+0.66%)	⬆️
Sources/Bitcoin/BitcoinPublicKey.swift	69.23% <100%> (ø)	⬆️
Sources/PrivateKey.swift	78.26% <100%> (ø)	⬆️
Tests/Bitcoin/BitcoinAddressTests.swift	100% <100%> (ø)	⬆️

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