Let EndEntityCert deref to Cert

[djc]

Avoids duplication and seems generally right. I think this would also make sense in the context of #174.

[codecov]

Codecov Report

Merging #184 (031099f) into main (dc9c85c) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   96.53%   96.52%   -0.01%     
==========================================
  Files          19       19              
  Lines        4496     4492       -4     
==========================================
- Hits         4340     4336       -4     
  Misses        156      156              

Files Changed	Coverage Δ
src/cert.rs	97.58% <40.00%> (ø)
src/end_entity.rs	100.00% <100.00%> (ø)
src/subject_name/verify.rs	91.84% <100.00%> (-0.05%)	⬇️
src/verify_cert.rs	97.35% <100.00%> (ø)

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[cpu]

This makes sense to me 👍