Add SmallTag type for more compact Dual types

[topolarity]

This is largely to improve printing and reduce visual noise for packages with large function types (e.g. SciML)

Compare tag = :small:

ForwardDiff.Dual{ForwardDiff.SmallTag{0x39f35d61d979c3d1}, Float64, 3}

to tag = :default:

ForwardDiff.Dual{ForwardDiff.Tag{var"#f#1"{DAEProblem{Vector{Float64}, Vector{Float64}, Tuple{Float64, Float64}, false, Vector{Float64}, DAEFunction{false, SciMLBase.FullSpecialize, typeof(f), Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, typeof(SciMLBase.DEFAULT_OBSERVED), Nothing, Nothing, Nothing}, @Kwargs{}, Vector{Bool}}, BrownFullBasicInit{Float64, Nothing}, DFBDF{5, 0, ADTypes.AutoForwardDiff{nothing, Nothing}, Nothing, OrdinaryDiffEqNonlinearSolve.NLNewton{Rational{Int64}, Rational{Int64}, Rational{Int64}, Rational{Int64}}, typeof(OrdinaryDiffEqCore.DEFAULT_PRECS), Val{:forward}(), true, nothing, Nothing, Nothing}}, Float64}, Float64, 3}