reorganized some distribution code
sampler interface, moved some mh options to fields
little improvements
switch from continuation marks to parameters for MH addresses
tidy example
add comment about context impl
remove unused exports
add partial impl of enumeration using iterative deepening

Summary: on the few toy/microbenchmarks I tried,
performance is same as with delimited continuations.
So, probably not worth keeping around; this commit will
be reverted.
Revert "add partial impl of enumeration using iterative deepening"

This reverts commit 7e785b2f8cf262cc44131443c94f3b2f88661112.
move rejection sampler