Add _id field in exported json

[olynch]

In order to produce julia-compatible json, I add an _id field to the exported json.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (45e5b2e) 87.38% compared to head (4ec0fdf) 87.46%.

Files	Patch %	Lines
src/acsets/acsets.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   87.38%   87.46%   +0.07%     
==========================================
  Files           6        6              
  Lines         325      327       +2     
  Branches       54       54              
==========================================
+ Hits          284      286       +2     
  Misses         36       36              
  Partials        5        5              

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