fix: traverser should skip over identity CIDs

[rvagg]

Same as #4 but against the proper branch

[codecov-commenter]

Codecov Report

Merging #6 (af79b20) into release/v0.13.x (bb8a59c) will increase coverage by 0.51%.
The diff coverage is 91.52%.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release/v0.13.x       #6      +/-   ##
===================================================
+ Coverage            63.28%   63.80%   +0.51%     
===================================================
  Files                   73       74       +1     
  Lines                 7951     8062     +111     
===================================================
+ Hits                  5032     5144     +112     
- Misses                2730     2732       +2     
+ Partials               189      186       -3     

Files Changed	Coverage Δ
testutil/testutil.go	82.46% <50.00%> (ø)
ipldutil/traverser.go	86.82% <53.84%> (-3.12%)	⬇️
testutil/identity.go	97.08% <97.08%> (ø)

... and 6 files with indirect coverage changes