Add a way to seed the thread local default generator

[John Nunley]

It would be nice to have a way to seed the RNG generator used for the module-level methods and for seeding the Rng structs by default. Maybe a seed() function or a FASTRAND_TLS_SEED environment variable.

[Taiki Endo]

a seed() function

See https://docs.rs/fastrand/latest/fastrand/fn.seed.html

[John Nunley]

Not sure how I missed that, thank you!