Implement iteration through certificate chain as an iterator.

[briansmith]

Factor out the boilerplate of navigating through the linked list.

[codecov]

Codecov Report

Merging #198 (c21d3b0) into main (1d5f4bd) will increase coverage by 0.19%.
The diff coverage is 69.76%.

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
+ Coverage   71.74%   71.94%   +0.19%     
==========================================
  Files          17       17              
  Lines        1405     1415      +10     
==========================================
+ Hits         1008     1018      +10     
  Misses        397      397              

Impacted Files	Coverage Δ
src/name/verify.rs	4.11% <0.00%> (ø)
src/verify_cert.rs	90.16% <85.71%> (+0.98%)	⬆️
src/cert.rs	96.35% <100.00%> (+0.55%)	⬆️
src/error.rs	72.22% <0.00%> (-14.45%)	⬇️

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