Generic-array should not be an explicit dependency

[jparr721]

Closes #<ISSUE_NUMBER>

This PR:

From the ticket:

Currently generic-array is an explicit direct dependency. All our use of generic-array is inherited from upstream RustCrypto crates such as blake3, sha3, digest, etc. Thus, the version of generic-array that we use should be whatever's used by these upstream crates. Thus, we should not list it as an explicit direct dependency.

Unfortunately, we need serde for generic-array and I don't know how to get it without listing generic-array as an explicit dependency. This is an insidious hack because it means that we must take care to always keep the version of generic-array we list in Cargo.toml consistent with whatever's used upstream.

This PR removes the generic-array dependency and instead gets it from digest which re-exports it.

This PR does not:

Key places to review:

[ggutoski]

Thanks! Looks like there's a CI failure to resolve but I'm happy.