fix(ffi): strict the length of rng seed bytes by hashing the digest

[hanyunx]

The uncertainty of the length of the seed bytes provided could trigger panic in the program. So this change enforces the strict length of 32 bytes of seeds - by hashing whatever amount of data provided into a 32-bytes seed.

Also some minor changes like comments typo.

[nategraf]

A couple of things here:

• I don't think we can actually change the RNG used in the blind function right away without breaking Valora. I'd recommend we implement this now, but use the deprecated method until we can check what impact this change will have on them. (It may not actually be a concern, but my current understanding is that it is)

  celo-threshold-bls-rs/crates/threshold-bls-ffi/src/wasm.rs

  Lines 382 to 405 in bb2bd53

  	// Creates a PRNG for use in deterministic blinding of messages or in key generation from the array
  	// of seeds provided as input.
  	fn get_rng(seeds: &[&[u8]]) -> impl RngCore {
  	let mut outer = Blake2s256::new();
  	outer.update("Celo POPRF WASM RNG Seed");
  	for seed in seeds.iter() {
  	outer.update(Blake2s256::digest(seed));
  	}
  	let seed = outer.finalize();
  	ChaChaRng::from_seed(seed.into())
  	}
  	// TODO(victor) Remove this when it is no longer used.
  	fn get_rng_deprecated(digest: &[u8]) -> impl RngCore {
  	let seed = from_slice(digest);
  	ChaChaRng::from_seed(seed)
  	}
  	fn from_slice(bytes: &[u8]) -> [u8; 32] {
  	let mut array = [0; 32];
  	let bytes = &bytes[..array.len()]; // panics if not enough data
  	array.copy_from_slice(bytes);
  	array
  	}

• We should try to start reducing the number of dependencies for these modules so they are not quite so big to include in mobile and web apps, where these are intended to be used. In celo-poprf-rs I implemented the get_rng function using the DirectHasher we already use elsewhere. I'd recommend we do that here so we can remove the blake2 dependencies. https://github.com/celo-org/celo-poprf-rs/blob/a5ea39fa2e5e282c5bd8ba0b333a5ea4e92755a5/src/ffi/wasm.rs#L509-L523
• We currently implement the get_rng function both in ffi.rs and wasm.rs. Probably best to extract it to lib.rs.

[nategraf]

Looks good to me! I left a couple of comments to look at, in addition to getting CI to pass, before this is merged.