test(types): Add mutation tests for ed25519

[Johennes]

This fixes:

MISSED   src/types/ed25519.rs:111:24: replace != with == in <impl Deserialize for ExpandedSecretKey>::deserialize in 0.8s build + 3.1s test
MISSED   src/types/ed25519.rs:153:9: replace Ed25519Keypair::expanded_secret_key -> Box<[u8; 64]> with Box::new([0; 64]) in 1.0s build + 3.7s test
MISSED   src/types/ed25519.rs:153:9: replace Ed25519Keypair::expanded_secret_key -> Box<[u8; 64]> with Box::new([1; 64]) in 0.9s build + 3.7s test
MISSED   src/types/ed25519.rs:209:9: replace Ed25519SecretKey::to_bytes -> Box<[u8; 32]> with Box::new([0; 32]) in 0.8s build + 2.8s test
MISSED   src/types/ed25519.rs:209:9: replace Ed25519SecretKey::to_bytes -> Box<[u8; 32]> with Box::new([1; 32]) in 0.9s build + 3.0s test
MISSED   src/types/ed25519.rs:214:9: replace Ed25519SecretKey::from_slice -> Self with Default::default() in 0.8s build + 2.8s test
MISSED   src/types/ed25519.rs:225:9: replace Ed25519SecretKey::to_base64 -> String with "xyzzy".into() in 0.9s build + 3.5s test
MISSED   src/types/ed25519.rs:225:9: replace Ed25519SecretKey::to_base64 -> String with String::new() in 0.9s build + 3.4s test
MISSED   src/types/ed25519.rs:235:24: replace != with == in Ed25519SecretKey::from_base64 in 0.8s build + 2.9s test
MISSED   src/types/ed25519.rs:235:47: replace && with || in Ed25519SecretKey::from_base64 in 0.8s build + 2.9s test
MISSED   src/types/ed25519.rs:235:62: replace != with == in Ed25519SecretKey::from_base64 in 0.8s build + 2.9s test
MISSED   src/types/ed25519.rs:235:9: replace Ed25519SecretKey::from_base64 -> Result<Self, crate::KeyError> with Ok(Default::default()) in 0.9s build + 3.7s test
MISSED   src/types/ed25519.rs:338:24: replace != with == in Ed25519PublicKey::from_base64 in 1.0s build + 3.0s test
MISSED   src/types/ed25519.rs:338:47: replace && with || in Ed25519PublicKey::from_base64 in 0.8s build + 2.9s test
MISSED   src/types/ed25519.rs:338:62: replace != with == in Ed25519PublicKey::from_base64 in 0.8s build + 3.0s test
MISSED   src/types/ed25519.rs:385:9: replace Ed25519PublicKey::verify -> Result<(), SignatureError> with Ok(()) in 0.9s build + 3.7s test
MISSED   src/types/ed25519.rs:404:9: replace <impl Display for Ed25519PublicKey>::fmt -> std::fmt::Result with Ok(Default::default()) in 0.8s build + 3.0s test
MISSED   src/types/ed25519.rs:448:9: replace <impl Display for Ed25519Signature>::fmt -> std::fmt::Result with Ok(Default::default()) in 0.8s build + 3.0s test
MISSED   src/types/ed25519.rs:70:9: replace ExpandedSecretKey::as_bytes -> &[u8; 64] with &[0; 64] in 0.8s build + 3.3s test
MISSED   src/types/ed25519.rs:70:9: replace ExpandedSecretKey::as_bytes -> &[u8; 64] with &[1; 64] in 0.8s build + 3.2s test

Relates to: #78

[poljar]

That's a nice chunk of tests. Thanks.