Better ability to hold non-Numbers

[jonniedie]

For things like DiffEqUncertainty.jl, it's important to hold things like Distributions alongside numbers in a parameter vector. Currently we are special-casing for only Nothing and Missing. There's no reason we shouldn't be able to hold things like Distributions, though. For example:

p_dist = ComponentArray(a=1.5, b=1.0, c=3.0, d=truncated(Normal(1.0,.1), .6, 1.4)])