(yarrow_slow_reseed): New function.
(yarrow256_update): Check seed/reseed thresholds.
(yarrow_gate): New function, extracted from
yarrow_generate_block_with_gate which was deleted.
(yarrow_generate_block_with_gate): Deleted function.
(yarrow256_random): Don't buffer any output, instead gate after
each request.

Rev: src/nettle/yarrow256.c:1.4