Fix bug in pvec pop resulting in empty tail

[georgematheos]

When calling pop on a persistent vector with, eg., 33 elements, we have a tail of length 1. After this pop call, there was a bug where the resulting pvec had a tail of length 0 rather than a tail of length 32. This causes errors, for instance when calling iterate, which expects a nonempty tail. This PR fixes the behavior by adding a branch in pop function for when the tail length is currently 1 (in which case we make the new tail be pop(trie)).

[georgematheos]

(just added a test for this behavior)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.38%. Comparing base (2dd9b36) to head (2bafd37).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   77.30%   78.38%   +1.07%     
==========================================
  Files           7        7              
  Lines         379      384       +5     
==========================================
+ Hits          293      301       +8     
+ Misses         86       83       -3     

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