Use is_human_readable methods with serde

[jplatte]

Currently, a cargo feature controls whether a MAC is (de)serialized as a formatted string or in binary. Since cargo features are resolved once for every dependency tree, it is impossible to use this crate for both formats in the same application, even through different crates (no direct dependency). serde has a helper method for determining whether the format should use a text or binary representation where both are possible, you should probably use that instead:

• Deserializer::is_human_readable
• Serializer::is_human_readable