Implement dummy types for distributions about soss.jl HOT 2 OPEN

Keep an eye out for stan-dev/stanc3#324 as well.

from soss.jl.

Right, currently this requires a For. I like the idea of being able to express it more simply, But to keep things simple and lightweight, I try to lean as much as possible on other libraries. To be able to write this, I would need to implement Poisson(::Array). That's not to say that this is out of the question, rather that it wouldn't need to be a core part of Soss.

As you point out, there's also the option of broadcasting. Poisson.(λ::Array) already creates an Array{Poisson}, so adding support for .~ would allow us to write

complaints .~ Poisson.(λ)

I'll comment more on this in #14.

from soss.jl.