Add missing method for varname_leaves; add tests

[penelopeysm]

varname_leaves didn't have a method for our best friend Cholesky. I need it in order to make penelopeysm/FlexiChains.jl#34 work nicely.

Also adds a series of tests for varname_leaves and varname_and_value_leaves.

Closes #138

[github-actions]

AbstractPPL.jl documentation for PR #141 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR141/

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.28%. Comparing base (5505e05) to head (fa8030f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/varname/leaves.jl	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   78.81%   86.28%   +7.46%     
==========================================
  Files           9        9              
  Lines         439      452      +13     
==========================================
+ Hits          346      390      +44     
+ Misses         93       62      -31     

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[penelopeysm]

I'm so confused, I thought I formatted.

Edit: Turns out I didn't have my local pre-commit config on this repo.