fix varname_..._leaves for other types

[penelopeysm]

This means that AbstractPPL / DynamicPPL don't fall over for variables with non-standard types. For example, if you have the value of @varname(s) being "a string" then calling varname_leaves on this value would crash, which this PR fixes.

Essentially this makes it possible (or perhaps less impossible, since I recognise there are also more problems in DynamicPPL and eltype of varinfo) to have values of arbitrary types in models (which I would point out is especially useful for :=).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.40%. Comparing base (f3c890f) to head (71583f9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   86.34%   86.40%   +0.05%     
==========================================
  Files           9        9              
  Lines         454      456       +2     
==========================================
+ Hits          392      394       +2     
  Misses         62       62              

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[github-actions]

AbstractPPL.jl documentation for PR #146 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR146/