Normalise optics when constructing VarName; remove extra constructors

[penelopeysm]

Closes #122.

I also added a changelog, because I've actually found it super helpful for DynamicPPL/Turing. I'm going to copy paste from it to describe this PR:

• Removed the constructors VarName(vn, optic) (this wasn't deprecated, but is IMO quite dangerous as it silently discards the existing optic in vn), and VarName(vn, ::Tuple) (which was deprecated).
• In the inner constructor of a VarName, its optic is now normalised to ensure that the associativity of ComposedFunction is always the same, and that compositions with identity are removed. This helps to prevent subtle bugs where VarNames with semantically equal optics are not considered equal.

Also changed all usage of ComposedOptic to ComposedFunction (it's the same thing: https://github.com/JuliaObjects/Accessors.jl/blob/18e9ddbc727ed4e48a3c19ffa00a02f7f407cdb5/src/optics.jl#L129).

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.56%. Comparing base (a39791b) to head (7bc83d8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/varname.jl	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   82.43%   83.56%   +1.12%     
==========================================
  Files           2        2              
  Lines         296      292       -4     
==========================================
  Hits          244      244              
+ Misses         52       48       -4     

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[coveralls]

Pull Request Test Coverage Report for Build 16098270412

Details

• 29 of 32 (90.63%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+1.1%) to 83.562%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/varname.jl	29	32	90.63%

Files with Coverage Reduction	New Missed Lines	%
src/varname.jl	16	78.09%

Totals
Change from base Build 16091588928:	1.1%
Covered Lines:	244
Relevant Lines:	292

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💛 - Coveralls

[github-actions]

AbstractPPL.jl documentation for PR #123 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR123/

[sunxd3]

lgtm

maybe add a comment somewhere saying that ComposedOptics is just a alias of ComposedFunctions? (I opted for ComposedOptics when migrating to Accessors because I thought it's simpler to understand in context of the migration. But I support use ComposedFunction now.)

[penelopeysm]

(I opted for ComposedOptics when migrating to Accessors because I thought it's simpler to understand in context of the migration. But I support use ComposedFunction now.)

That makes sense. I guess it's really Julia that had me confused 😄 , since Accessors writes const ComposedOptic = ComposedFunction whenever you dump something it prints as ComposedFunction.

julia> using Accessors

julia> @o _.a.b
(@o _.a.b)

julia> dump(ans)
(@o _.a.b) (function of type ComposedFunction{PropertyLens{:b}, PropertyLens{:a}})
  outer: PropertyLens{:b} (@o _.b)
  inner: PropertyLens{:a} (@o _.a)

I'll add a note!