encapsulate logjac information in result

[tpapp]

The methods in this package keep track of absolute log Jacobian determinants by returning an extra value. This is somewhat brittle, I am proposing an alternative way of doing it.

Let

import Distributions: logpdf

struct LogJacVariate{T,S}
    x::T
    logjac::S
end

logpdf(d, v::LogJacVariate) = logpdf(d, v.x) + v.logjac

Then the API that replaces transform_and_logjac would just return a LogJacVariate.

Caveats: Distributions.jl is a heavy dependency. Factoring out the core, as proposed eg in JuliaStats/Distributions.jl#1139, would make this much easier.