Promote eltypes on construction instead of defaulting to Float64

[jonniedie]

Right now this happens:

julia> x = ComponentArray(a=1, b=2f0)
ComponentVector{Float64}(a = 1.0, b = 2.0)

when this should happen:

julia> x = ComponentArray(a=1, b=2f0)
ComponentVector{Float32}(a = 1.0, b = 2.0)

We also want this (which currently errors):

julia> x = ComponentArray(a=1, b=missing)
ComponentVector{Union{Missing, Int64}}(a = 1, b = missing)

julia> x = ComponentArray(a=1, b=2+im)
ComponentVector{Complex{Int64}}(a = 1 + 0im, b = 2 + 1im)
``