add sort.rkt
sort and combine duplicates in discrete-dist
fix missing keyword in contract
cache normalizing constants for dirichlet dists
update comments
add transition for slice sampling
make mh-transitions easier to use

rename hamiltonian-mc -> hmc
move MH threshold handling; debug info for transitions

move MH acceptance to transition base class
each transition object keeps its own statistics
moved long source comments to internals docs
fix bug in stats
add note to example re interpretation
update todo
add improper-dist, factor
MH fix: if first sample fails, retry (don't return #f)
added prob/matrix
added multi-normal-dist, more matrix utils
split out prob/private/matrix-*, more matrix utils
add wishart-dist, split out prob/private/dist-impl
prob/matrix: wrap more operations, add for/matrix syntax
added naive impl of inv-wishart-dist