factored out dist module

renamed old "discrete-dist" to "categorical-dist"; see comments
exact bernoulli dist
... bug in bernoulli
add dist stats (eg support, mean, variance)
... checkpoint
... work on discrete-dist
... checkpoint
...
...
move most dist code to prob/dist
update priors using conjugacy
sampler interfaces, classes
use generic defaults
dirichlet dist
... fix samples to fit new interfaces
dirichlet conjugate prior
pareto dist
added #:normalize? arg to discrete-dist funs
cleanup
add dist docs

also, fix normal conjugage (stddev, not variance)