Pass model via Base.getindex

[olynch]

Keyword arguments don't play well with dispatch, and we don't want the model to be the first parameter when unwrapped by WithModel because models might be elements of another model, so instead we pass in the model via stuff like id[FinSetC()](3).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (7f93f97) 96.82% compared to head (6d791a3) 96.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
- Coverage   96.82%   96.82%   -0.01%     
==========================================
  Files          25       25              
  Lines        1449     1447       -2     
==========================================
- Hits         1403     1401       -2     
  Misses         46       46              

Files Changed	Coverage Δ
src/stdlib/models/FinMatrices.jl	100.00% <ø> (ø)
src/stdlib/models/FinSets.jl	92.30% <ø> (ø)
src/stdlib/models/GATs.jl	100.00% <ø> (ø)
src/stdlib/models/Nothings.jl	100.00% <ø> (ø)
src/models/ModelInterface.jl	97.63% <100.00%> (ø)
src/stdlib/models/SliceCategories.jl	88.88% <100.00%> (ø)
src/syntax/TheoryInterface.jl	97.87% <100.00%> (-0.09%)	⬇️

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