Type stability when having function as struct field

[jling]

@yhchang96

@with_kw struct ParDemo1{T1<:Union{Float64,Function}}
    D::Float64 = 5.0
    A::Float64 = 0.0
    W::Float64 = 1.0
    V::T1 = 1.0
    ℿ::Function = ℿdemo
end

@with_kw struct ParDemo1{T1<:Union{Float64,Function}, F}
    D::Float64 = 5.0
    A::Float64 = 0.0
    W::Float64 = 1.0
    V::T1 = 1.0
    ℿ::F = ℿdemo
end