Support JSON lines in load/dumpfn

[esoteric-ephemera]

• Support streaming I/O of JSON lines files as an extension in loadfn and dumpfn
• Tests for new behavior
• Test fixture for running in temp dir, removes boilerplate cleanup code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.36%. Comparing base (a0a2265) to head (64636e4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #770      +/-   ##
==========================================
+ Coverage   85.27%   85.36%   +0.08%     
==========================================
  Files          27       27              
  Lines        1664     1667       +3     
  Branches      315      315              
==========================================
+ Hits         1419     1423       +4     
  Misses        186      186              
+ Partials       59       58       -1     

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