Implement FromDer trait and DerIterator type

[djc]

Right now this is only a little bit more concise but I feel like it might help structure things going forward?

Needs a thorough audit for the correctness of consuming entire DER sequences, not sure we're entirely consistent (and have good coverage of) the read_all() aspect of things. (But I think this could help get that right in the future?)

[codecov]

Codecov Report

Merging #139 (0c796ab) into main (55f7b5d) will decrease coverage by 0.32%.
The diff coverage is 100.00%.

❗ Current head 0c796ab differs from pull request most recent head 41c6d31. Consider uploading reports for the commit 41c6d31 to get more accurate results

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
- Coverage   95.35%   95.04%   -0.32%     
==========================================
  Files          15       15              
  Lines        4007     3731     -276     
==========================================
- Hits         3821     3546     -275     
+ Misses        186      185       -1     

Files Changed	Coverage Δ
src/verify_cert.rs	95.23% <ø> (ø)
src/cert.rs	96.66% <100.00%> (-0.14%)	⬇️
src/crl.rs	100.00% <100.00%> (+0.33%)	⬆️
src/der.rs	98.14% <100.00%> (+0.04%)	⬆️
src/signed_data.rs	100.00% <100.00%> (ø)
src/subject_name/verify.rs	96.56% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[cpu]

This looks like a nice tidy-up to me, thanks @djc!

Tiny nit: maybe squash the 3rd commit into the 1st and switch the usages over to the new pattern in one fell swoop?

[djc]

Needs a thorough audit for the correctness of consuming entire DER sequences, not sure we're entirely consistent (and have good coverage of) the read_all() aspect of things. (But I think this could help get that right in the future?)

I spent some time looking at this and I think it's now correct. But review on these aspects would be good!

Tiny nit: maybe squash the 3rd commit into the 1st and switch the usages over to the new pattern in one fell swoop?

Yup, good point -- done.

[ctz]

looks reasonable to me!

[djc]

@cpu hope this doesn't cause too much thrashing in your open PRs!

[cpu]

Nothing some git-fu won't solve :-)