memoize some matrix ops
update todo
improve comments & internal docs on record-obs?
turn off record-obs? by default
MH: add {set,push,pop}-transition
MH: add rerun
MH: lift common {single,multi}-site transition code to abs class
update dist docs, fix related minor issues
update docs on primitive (vs derived) stochastic features
update samplers/solvers docs
change enumerate default #:limit to #f
fix enum-importance-sampler bugs
work on utils docs
updated statistics to work with reals and col-matrixes
allow disabling printing of matrix wrapper struct

Not runtime configurable (grr), and disabled for now
(until matrix code settles down).
force ImmArray-wrapped arrays to be immutable
update misc utils docs
add matrix docs, reexport prob/matrix from prob/main
add docs for particles, move contracts to main
add particles-score convenience function