Refactor DER parsing

[djc]

Now that we no longer depend on generic ring things, there is a bunch of stuff we can do to simplify DER handling.

[codecov]

Codecov Report

Merging #142 (69441d6) into main (aa818ab) will increase coverage by 0.22%.
Report is 3 commits behind head on main.
The diff coverage is 98.97%.

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   96.01%   96.24%   +0.22%     
==========================================
  Files          15       15              
  Lines        4065     4259     +194     
==========================================
+ Hits         3903     4099     +196     
+ Misses        162      160       -2     

Files Changed	Coverage Δ
src/error.rs	50.00% <50.00%> (+1.06%)	⬆️
src/time.rs	96.15% <96.29%> (+0.09%)	⬆️
src/cert.rs	96.91% <98.82%> (+0.12%)	⬆️
src/crl.rs	100.00% <100.00%> (+0.34%)	⬆️
src/der.rs	99.57% <100.00%> (+0.20%)	⬆️
src/end_entity.rs	100.00% <100.00%> (ø)
src/ring_algs.rs	100.00% <100.00%> (ø)
src/signed_data.rs	98.72% <100.00%> (-0.46%)	⬇️
src/subject_name/verify.rs	92.39% <100.00%> (+0.30%)	⬆️
src/trust_anchor.rs	96.82% <100.00%> (+0.59%)	⬆️
... and 2 more

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[cpu]

Thanks, this looks good to me. I have some suggestions for the error variant names but I don't feel strongly if you disagree with the suggestions I've made.

[djc]

Okay, Error::TrailingData now contains a DerTypeId, and FromDer has a TYPE_ID associated const.