all: use OsRng instead of thread_rng()

[conradoplg]

While our libraries don't do RNG directly and take an CryptoRng from the caller, we do have a bunch of tests and examples that do RNG and that people will inevitably copy.

Reading rands CHANGELOG while reviewing #853, I noticed that it has a new "rand is not a crypto library" policy. Its thread_rng that we were using is also not fork-safe. Looking for alternatives I saw that librustzcash uses OsRng, which seems to be the right thing to do (even if it's still in rand, but it's basically a getrandom() wrapper that implements CryptoRng, which is exactly what we need).

This changes all usages of thread_rng() to use OsRng instead.