Stop using sodiumoxide

[abacabadabacaba]

We are currently using libsodium (via sodiumoxide crate). I see a number of reasons against it:

• libsodium is written in C and doesn't fully conform to Rust conventions. For example, we have to call init() (do we?), or some of its functions will not be thread-safe.
• libsodium only provides a high-level API. We cannot use it to implement a VRF (New VRF implementation #1653), or randomness (Randomness NEPs#22), or anything else that requires low-level cryptography.
• We only use a small subset of libsodium's functionality. When I searched through the code, I only found uses of SHA-256 hash function and Ed25519 signature scheme. Yet, we compile and link the entire library.

I propose to use these crates instead of sodiumoxide:

• For SHA-256: sha2. Also, consider switching to a more modern hash function, such as BLAKE2.
• For Ed25519: ed25519-dalek.

[ailisp]

I'm going to ensure every c/c++ dependency doesn't use too new instruction set (which cause some validator with old cpus report: illegal instructions). This is better done before examine them so I'll take this