[feat] add support for serde

[XieJiSS]

Scenario: One may want to deserialize into a HashMap<String, String> from, say, a config file containing a map with arbitrary keys. There's a config-rs crate for this, but due to rust-cli/config-rs#531, the deserialized keys are always converted to lowercase. The consequence is catastrophic: for a Map<user_id, priority> where user_id might contain upper cased letters, anyone forgetting to add a .to_ascii_lowercase() will find None from the hash map.

Many crates also have a special serde feature for serialize/deserialize using serde_json or other similar libraries built upon the serde crate. So maybe we should add this feature into unicase as well.

[XieJiSS]

Sorry, didn't notice that there's already #41. Will close this dup PR.