basic functionality moved to counter_traits class
clean up ars_example and m128_traits.  Seems to work
prf_common.hpp is obsolete
UintType is now the first (required) template param for counter_based_engine
get rid of preferred_result_type
domain_error -> invalid_argument
discourage counter_traits::make_counter(SeedSeq)
add initializer_lists to restart.  make_counter_from_seed_seq is private
make_counter(const It&, It)
counter_traits::make_counter()
call it CtrType instead of a_type