Support ed25519 signature verification (e.g. Tendermint block headers)

[ethanfrey]

Part of #751
Follow same steps as #752

Add a function verify_ed25519() to MockApi to verify ed25519 signatures. This should take the raw pubkey and signature bytes as used in tendermint core to sign blocks.

There seems to be much more standardization in ed25519, so we hopefully only need this one variant, not 3 different encodings as with secp256k1 (cosmos, ethereum, bitcoin).

[ethanfrey]

As mentioned in #751 (comment) let us add a batch verify here as well.

It should be something like batch_verify(message: &[u8], pubkeys: &[&[u8]], signatures &[&[u8]]) -> Vec<bool> or bool (either return results of which ones are valid, or just if all are valid.

pubkeys and signatures are both N elements long (must be the same). If the return value is Vec<bool>, it must also be N elements long. Do we send the original message or the message hash like with secp256k1? I guess that is the same as whatever you do for the single verify function (which should be done first).

[ethanfrey]

Request was to use https://crates.io/crates/ed25519-zebra that implements this
This is also planned for CosmJS support: cosmos/cosmjs#584
And is the 100% compatible crate for tendermint now: tendermint/tendermint#5632

[webmaster128]

Do we send the original message or the message hash like with secp256k1

Ed25519 supports arbitrary input lengths because the algorithm itself has an internal hashing step. In secp256k1 the input must not exceed 32 bytes. This is why you use an external hashing step. As a consequence, the secp256k1_verify takes a message_hash and ed25519_verify takes a message.