partial fix for permutation-pdf
Fix HMC samper contract and HMC example.
add optional args for identity-matrix
add generic interface for drift proposals
move multivariate dist impls, add drift to multi-normal
move univariate dist impl code
move discrete-dist impl code
remove useless requires
compute log pdf for univariate dists
compute log pdf for multivariate dists
add logspace+, logspace-sum functions
add slice-adjust generic function

reduces MH dependencies on prob/dist
fix types for ->{row,col}-matrix

Add handling for (U t Array) types, and
add arity overlap checking.