Provide a `SmolStr` with a lifetime

[Jason5Lee]

Provide a SmolStr<'a> type, which is implemented exactly the same as the current one. The only difference is that all functions converting from &'static str will become converting from &'a str, resulting in a 'a lifetime, while other cases result in a 'static lifetime. This makes it more flexible to use.

Use case 1: Implement from_utf8_lossy for SmolStr.

fn smolstr_from_utf8_lossy(v: &[u8]) -> SmolStr<'_> {
    // Simplified pseudocode
    if let Some(s) = v as valid utf-8 &str {
       SmolStr::from_str(s)
    } else {
        let mut res = SmolStrBuilder::new();
        build result string
        res.finish().as_static_lifetime()
    }
}

Use case 2: Serialization / Deserialization

#[derive(Serialize, Deserialize)]
struct Data<'a> {
    field: SmolStr<'a>,
}

// Serialize: field can be a borrow of a certain lifetime.
fn serialize(field: &str) -> String {
    serde_json::to_string(&Data { field: SmolStr::from_str(field) })
}

// Deserialize: get static lifetime, an inline or heap-allocated SmolStr underneath
fn deserialize(json: &str) -> serde_json::Result<Data<'static>> {
    serde_json::from_str(json)
}