added require utils.random
typo bugfixes
defined rand-idx via defmulti, commented out sfmt

SFMT19937 is not worth the trouble (at this point, anyway).
MersenneTwister is good, and SFMT19937 only provides a next int
function, where the int can range over the entire range of Integer.
Trying to convert this into an int between 0 and n can be done,
but why bother?  I don't even know whether the number is in
[Integer/MIN_VALUE, Integer/MAX_VALUE), or some other way of dealing
with the endpoints.  I want [...) for next-idx.
sample fns now take rng rather than rand-idx args

and rand-idx is defined by a defmulti to always take an rng arg.
(If you don't want that, use partial.)

also got rid of make-...from-next-double
Added next-long
Fixed rng-setting with make-rng and next-long
cruft from recent edits of other files
replaced next-int in sample calls with rng from person
bugfix added actual rng field to Person