Update rand_core requirement from 0.5.1 to 0.6.4

Updates the requirements on [rand_core](https://github.com/rust-random/rand) to permit the latest version.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@rand_core-0.5.1...0.6.4)

---
updated-dependencies:
- dependency-name: rand_core
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.toml

@@ -10,6 +10,6 @@ readme = "README.md"
 
 [dependencies]
 rand = "0.7.3"
-rand_core = "0.5.1"
+rand_core = "0.6.4"
 sha3 = "0.9.1"
 curve25519-dalek = "3.2.1"