Split up varname.jl into more manageable bits

[penelopeysm]

Closes #126

Also removes coveralls, and updates actions/checkout from v4 to v5

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.81%. Comparing base (4c77999) to head (d274a95).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/varname/prefix.jl	86.66%	4 Missing ⚠️
src/varname/subsumes.jl	93.75%	3 Missing ⚠️
src/varname/serialize.jl	98.14%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #140   +/-   ##
=======================================
  Coverage   78.81%   78.81%           
=======================================
  Files           6        9    +3     
  Lines         439      439           
=======================================
  Hits          346      346           
  Misses         93       93           

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[github-actions]

AbstractPPL.jl documentation for PR #140 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR140/

[coveralls]

Pull Request Test Coverage Report for Build 18245953691

Details

• 124 of 132 (93.94%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.815%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/varname/serialize.jl	53	54	98.15%
src/varname/subsumes.jl	45	48	93.75%
src/varname/prefix.jl	26	30	86.67%

Totals
Change from base Build 17351053994:	0.0%
Covered Lines:	346
Relevant Lines:	439

---

💛 - Coveralls