convert to UnionAll types

[goretkin]

I expected

julia> convert(Vector{T} where T <: AbstractFloat, [1, 2])
ERROR: MethodError: no method matching (Vector{A} where A<:AbstractFloat)(::Vector{Int64})

to work, computing e.g. convert.(AbstractFloat, [1, 2])

On zulip another case was brought up

julia> convert(Complex{AbstractFloat}, 1+im)
1.0 + 1.0im

julia> convert(Complex{T} where T <: AbstractFloat, 1+im)
ERROR: MethodError: no method matching (Complex{T} where T<:AbstractFloat)(::Complex{Int64})

I understand that it is tricky to support UnionAll, but the need arises in my own packages and it would be nice to fall back on some examples in Base.