Initial commit for distribution package. Contains Exponential, Laplace, ...

...Normal, and Uniform
@kortschak is allocating + returning newPriorValue and newPriorWeight a mistake? Should the values instead be overwritten in place, and have the user copy the data if they want to keep it? I originally had them returned because I wanted to allow using nil for the priors, but the user can also just have a priorWeight of zero for the same effect.