The standard RNG. The PRNG algorithm in StdRng is chosen to be efficient
on the current platform, to be statistically strong and unpredictable
(meaning a cryptographically secure PRNG).
Retrieve the lazily-initialized thread-local random number generator,
seeded by the system. Intended to be used in method chaining style,
e.g. thread_rng().gen::<i32>(), or cached locally, e.g.
let mut rng = thread_rng();. Invoked by the Default trait, making
ThreadRng::default() equivalent.