Use one PRNG even if user requests multiple passphrases

[sts10]

In my amateur research of CSPRNGs for #21, I came across some cryptographic notes of caution about generating many pseudo-random number generators (I'll try to find some of these links).

The current version of Phraze creats a CSPRNG for each passphrase the user requests. So for example, if the user runs phraze -n 5 to get 5 passphrases, Phraze creates 5 CSPRNGs, one for each.

In contrast, this PR has Phraze create one CSPRNG in main.rs and use that to generate all passphrases that the user might ask for.

I'm not yet sure which approach is more safe/secure, but like #21, I'm opening a PR as a question for research/comments/thoughts. Also, of course, it might not really be significant for Phraze.

[sts10]

This section from the Rust Rand Book on Parallel RNGs seems to imply that it's fine to use "a custom RNG per worker thread" or "per work unit". If we assume it's safe to think of each requested passphrase as a "work unit", Phraze existing approach of creating a "custom" generator for each passphrase is inline with a "suggested approach" from the Rand Book, which is good enough for me.

[sts10]

If we do end up doing this, I should move the PRNG creation to src/lib.rs, but we'll cross that bridge when we get there.

[sts10]

I think using multiple PRNGs is fine, from a security-perspective. Closing this without merging at this time.