Improve type stability

[cscherrer]

This PR makes some adjustments that aim to improve type stability. Both JET.jl and Aqua.jl seem happier after these changes. I didn't do exhaustive performance testing, but as one proxy for this...

# Before this PR
julia> @time Pkg.test()
 17.379904 seconds (264.21 k allocations: 24.215 MiB, 0.07% gc time)

# After this PR
julia> @time Pkg.test()
 13.845315 seconds (265.68 k allocations: 24.159 MiB, 0.08% gc time)
end

[codecov-commenter]

Codecov Report

Merging #12 (8c05a67) into main (2fed970) will increase coverage by 0.14%. The diff coverage is 93.93%.

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@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   94.02%   94.17%   +0.14%     
==========================================
  Files           6        6              
  Lines         201      206       +5     
==========================================
+ Hits          189      194       +5     
  Misses         12       12              

Impacted Files	Coverage Δ
src/core.jl	84.84% <89.47%> (+1.51%)	:arrow_up:
src/autorun.jl	100.00% <100.00%> (ø)
src/effecthandler.jl	94.11% <100.00%> (-0.33%)	:arrow_down:
src/instances.jl	97.46% <100.00%> (+1.21%)	:arrow_up: