Consider allowing borrowed data in the provided data types

[Marwes]

Since all the types in this crate only hold String this library copies more data than it needs it some situations (both serializing and deserializing). Probably the RPC overhead will make these copies insignificant in most cases, but this might be a micro optimization worth doing (measure first of course).

Two ways of doing this.

Replace String fields with Cow<str>. Easy to do but slightly more difficult to use when one do not care about copies.

Parameterize structs containing strings by the string type used.

pub struct Command {
    /// Title of the command, like `save`.
    pub title: String,
    /// The identifier of the actual command handler.
    pub command: String,
    /// Arguments that the command handler should be
    /// invoked with.
    #[serde(skip_serializing_if="Option::is_none")]
    pub arguments: Option<Vec<Value>>,
}
pub struct Command <S = String> {
    /// Title of the command, like `save`.
    pub title: S,
    /// The identifier of the actual command handler.
    pub command: S,
    /// Arguments that the command handler should be
    /// invoked with.
    #[serde(skip_serializing_if="Option::is_none")]
    pub arguments: Option<Vec<Value>>,
}

Setting String as the default value may make this less of a breaking change but I am not sure how much it helps. This would make it possible to use Command<&str> or Command<Cow<str>>.