Fix CRC32 check and introduce test and testzip methods

[miurahr]

related #137

[coveralls]

Coverage remained the same at 83.569% when pulling 7177a1d on topic-digest-check into 7b801d3 on master.

[codecov]

Codecov Report

Merging #138 into master will increase coverage by 0.11%.
The diff coverage is 75.55%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   85.32%   85.44%   +0.11%     
==========================================
  Files          12       12              
  Lines        2665     2666       +1     
  Branches      444      443       -1     
==========================================
+ Hits         2274     2278       +4     
+ Misses        257      256       -1     
+ Partials      134      132       -2     

Flag	Coverage Δ
#linux	81.73% <75.55%> (+0.11%)	⬆️
#mac	81.32% <75.55%> (+0.11%)	⬆️
#py36	83.87% <75.55%> (+0.11%)	⬆️
#py37	84.89% <75.55%> (+0.11%)	⬆️
#py38	84.78% <75.55%> (+0.11%)	⬆️
#pypy3	79.38% <75.55%> (+0.12%)	⬆️
#windows	83.64% <75.55%> (+0.11%)	⬆️

Impacted Files	Coverage Δ
py7zr/cli.py	84.06% <0.00%> (ø)
py7zr/py7zr.py	85.11% <71.42%> (+0.23%)	⬆️
py7zr/compression.py	94.29% <81.81%> (-0.26%)	⬇️
py7zr/__init__.py	69.23% <100.00%> (ø)
py7zr/archiveinfo.py	83.62% <100.00%> (+0.13%)	⬆️
py7zr/exceptions.py	100.00% <100.00%> (ø)