Refactor name verification flow

[djc]

Moves code around to reduce coupling and introduce clearer abstractions. Preamble for work on #183.

[codecov]

Codecov Report

Merging #188 (8fef816) into main (12fe2bb) will decrease coverage by 0.03%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
- Coverage   96.52%   96.50%   -0.03%     
==========================================
  Files          19       19              
  Lines        4492     4466      -26     
==========================================
- Hits         4336     4310      -26     
  Misses        156      156              

Files Changed	Coverage Δ
src/subject_name/dns_name.rs	87.95% <90.47%> (-0.71%)	⬇️
src/subject_name/ip_address.rs	98.29% <94.73%> (-0.09%)	⬇️
src/end_entity.rs	100.00% <100.00%> (ø)
src/subject_name/verify.rs	91.11% <100.00%> (-0.74%)	⬇️

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[cpu]

LGTM 👍