LogDensityFunction should be exported

[penelopeysm]

I think we use it upstream quite a bit. and also I have in mind that for some of the sampler/modelling refactoring what we really want to do is to implement methods like

sample(::LogDensityFunction, ::MySampler, N)

(note that sampling from a LogDensityFunction right now is surprisingly non-trivial, it has to be wrapped yet again in a AbstractMCMC.LogDensityModel or something like that)

and have

sample(::Model, ::MySampler, N) forward to that.

It would be possible, but very annoying to do this if LogDensityFunction isn't exported, so I think we should export it.

[yebai]

We could consider merging DynamicPPL.LogDensityFunction and AbstractMCMC.LogDensityModel.