Extract PathNode from Cert

[djc]

Previously, a Cert type (which mostly represents a certificate) also doubled as a linked list node via the (somewhat cryptically named) ee_or_ca field. Instead, use a linked list node type (here called PathNode to evoke the somewhat well-known notion of path building) that contains the Cert.

Cert construction becomes a lot more obvious and simple because it is no longer obliged to deal with the linked list, and we can draw a clean distinction between types for dealing the with the linked list path and types that just need the certificate data from a Cert.

After spending some time with the path-building code, I wanted to try and improve cohesion so tacked on a bunch of commits that try to colocate related code.

[codecov]

Codecov Report

Merging #173 (8c68276) into main (7ff3664) will increase coverage by 0.05%.
Report is 1 commits behind head on main.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##             main     #173      +/-   ##
==========================================
+ Coverage   96.64%   96.70%   +0.05%     
==========================================
  Files          16       17       +1     
  Lines        4532     4518      -14     
==========================================
- Hits         4380     4369      -11     
+ Misses        152      149       -3     

Files Changed	Coverage Δ
src/crl/mod.rs	97.42% <97.42%> (ø)
src/verify_cert.rs	97.79% <97.60%> (+0.11%)	⬆️
src/crl/types.rs	99.53% <99.55%> (ø)
src/cert.rs	97.47% <100.00%> (+0.60%)	⬆️
src/end_entity.rs	100.00% <100.00%> (ø)
src/subject_name/verify.rs	96.27% <100.00%> (-0.14%)	⬇️
src/trust_anchor.rs	98.48% <100.00%> (ø)

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[cpu]

Cool, this worked out nicely 👍

New revisions

[cpu]

Really appreciate this clean-up 🙏 Thanks!