* rands: use splitmix64 for seeding
Seeding with splitmix64 is a good way to avoid starting with
low-entropy PRNG states, and is explicitly recommended
by the authors of both xoshiro256++ and Romu.
While at it, give the xoshiro256++ PRNG its proper name.
* rands: use fast_bound() to generate number in range
* rands: add top-level choose()
* rands: add Rand::next_float()
* rands: add Rand::coinflip() helper
* libafl: unbreak tests that relied on direct seeding
* rands: add SFC64 PRNG
SFC64 is a well-established and well-understood PRNG designed by
Chris Doty-Humphrey, the author of PractRand. It has been tested
quite a lot over the years, and to date has no known weaknesses.
Compared to xoshiro256++, it is slightly faster and is likely to
be a more future-proof design (xoshiro/xoroshiro family of generators
come with quite long history of [flaws][1] found over the years).
Compared to Romu, it is slightly slower, but guarantees absense
of bias, minimum period of at least 2^64 for any seed, and
non-overlapping streams for different seeds.
[1]: https://tom-kaitchuck.medium.com/designing-a-new-prng-1c4ffd27124d
