Micro-optimise append_optic; fix serialisation for JSON@1.5.0

[penelopeysm]

shortcircuit the recursion in a simple case

[github-actions]

AbstractPPL.jl documentation for PR #154 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR154/

[penelopeysm]

honestly this is not even a micro-optimisation, it's more like a nano-optimisation

the CI failure is bizarre though, i can't reproduce it offline Reproduced, the culprit is upgrading JSON.jl from 1.4.0 to 1.5.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.51%. Comparing base (97d0f21) to head (0211966).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
+ Coverage   84.49%   84.51%   +0.02%     
==========================================
  Files          10       10              
  Lines         561      562       +1     
==========================================
+ Hits          474      475       +1     
  Misses         87       87              

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