Replace `prob` macro with `SimpleVarInfo`

[yebai]

See #309 (comment)

IMO this should be removed. Once we have SimpleVarInfo, this is completely redundant.

Similarly we should IMO remove the prob"..." macro, since now it will be possible to add something like

function logdensity(model::Model, theta::NamedTuple)
    return logdensity(model, SimpleVarInfo(theta))
end

It might be safer to depreciate logprob with a suggestion to use logdensity first. Also, we need to update the documentation on prob/logprob.

Related: #315 (comment) #325 (comment)

See also the | notation for models.

[yebai]

@sunxd3 can you take a look at this issue and see whether we can safely remove prob in favour of logprior, logjoint and loglikelihood functions?

[torfjelde]

I'm very, very in favour of this! It's about time we deprecate the logprob macro.