Various win_cert improvements

[nicholasbishop]

• Add more tests
• Add size field to invalid-cert-size error
• Better error handling
• More comments

[codecov-commenter]

Codecov Report

Merging #39 (f2732a7) into main (53a1fde) will increase coverage by 1.72%. The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   87.27%   88.99%   +1.72%     
==========================================
  Files           7        7              
  Lines         495      509      +14     
==========================================
+ Hits          432      453      +21     
+ Misses         63       56       -7     

Files	Coverage Δ
authenticode/src/win_cert.rs	95.57% <93.75%> (+7.69%)	:arrow_up: