Clarify deletion of variables

[blegat]

Extracted from #759. It also refactors implementation of names to share more code with UniversalFallback that will also be used by bridges in #759

Closes #511

[mlubin]

I reviewed only the docs, haven't looked at the code yet.

[codecov-io]

Codecov Report

Merging #793 into master will increase coverage by 0.06%.
The diff coverage is 96.93%.

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
+ Coverage   93.99%   94.06%   +0.06%     
==========================================
  Files          59       59              
  Lines        6542     6702     +160     
==========================================
+ Hits         6149     6304     +155     
- Misses        393      398       +5

Impacted Files	Coverage Δ
src/Utilities/sets.jl	100% <ø> (ø)	⬆️
src/indextypes.jl	85.71% <ø> (ø)	⬆️
src/Test/modellike.jl	99.22% <100%> (+0.17%)	⬆️
src/Utilities/results.jl	80.66% <100%> (ø)	⬆️
src/Utilities/functions.jl	95.08% <100%> (ø)	⬆️
src/Bridges/Constraint/slack.jl	93.54% <100%> (ø)	⬆️
src/sets.jl	89.65% <33.33%> (-3.08%)	⬇️
src/Utilities/mockoptimizer.jl	90.14% <85.71%> (-0.16%)	⬇️
src/Utilities/universalfallback.jl	97.29% <96%> (-0.25%)	⬇️
src/Utilities/model.jl	93.22% <98.27%> (+1.75%)	⬆️
... and 2 more

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