It was a mainly a legacy from when we were going via the old rt to read the initial seed from the OS. The reason for it is to make `{IsaacRng, Isaac64Rng}::new` neater, by sharing the code for generating the initial seeds.

However, it is "clearly" bad, since (e.g.) it's so unsafe, and the allocation is unnecessary, it should just be `.fill_bytes`-ing the internal buffer of the generators.