Minor bug- and clippy fixes

[kpcyrd]

A very brief list of what was changed:

• In KeyType::from_ed25519_with_keyid_hash_algorithms it's using &key[..ED25519_PRIVATE_KEY_LENGTH] and then &key[ED25519_PUBLIC_KEY_LENGTH..], while the "correct" thing would be &key[ED25519_PRIVATE_KEY_LENGTH..] to refer to the bytes after the private key. This ultimately doesn't matter at all in the final binary though, since both constants are 32. This patch changes it to let (private_key_bytes, public_key_bytes) = key.split_at(ED25519_PRIVATE_KEY_LENGTH);
• In the error message for unknown oid's, it's encoding each byte in a byte array to hex using {:x}, this works as expected for 0x10 to 0xff, but for values like 0x00 to 0x0f it would print 0 and f respectively. The byte sequence [0x0f, 0x77, 0x0f] would get hex encoded as f77f, suggesting [0xf7, 0x7f]. This is inside of an error branch though and doesn't have any real impact on the library.
• A few match bool { true => {}, false => {} }; expressions have been rewritten to if bool { ... } else { ... } or .then_some(...).
• Instead of impl AsRef<Vec<String>> for Command { do impl AsRef<[String]> for Command { because with an immutable reference, a Vec (pointer + length + capacity) doesn't have any benefits over a slice (pointer + length). This is technically a semver breaking change though.
• Instead of x if x == ED25519_SPKI_OID => Ok(KeyType::Ed25519), Rust supports writing just ED25519_SPKI_OID => Ok(KeyType::Ed25519),

[alanssitis]

LGTM

[adityasaky]

Thanks @kpcyrd!