Support for DensityInterface

Distributions supports DensityInterface for distributions.

A probability distribution has a probability density, so DensityInterface.DensityKind(::Distribution) === HasDensity().

For single variate values x, DensityInterface.logdensityof(d::Distribution, x) is equivalent to logpdf(d, x) and DensityInterface.densityof(d::Distribution, x) is equivalent to pdf(d, x).