Use Box<str> instead of String for User

telers/src/types/user.rs

@@ -20,13 +20,13 @@ pub struct User {
     /// `true`, if this user is a bot
     pub is_bot: bool,
     /// User's or bot's first name
-    pub first_name: String,
+    pub first_name: Box<str>,
     /// User's or bot's last name
-    pub last_name: Option<String>,
+    pub last_name: Option<Box<str>>,
     /// User's or bot's username
-    pub username: Option<String>,
+    pub username: Option<Box<str>>,
     /// [`IETF language tag`](https://en.wikipedia.org/wiki/IETF_language_tag) of the user's language
-    pub language_code: Option<String>,
+    pub language_code: Option<Box<str>>,
     /// `true`, if this user is a Telegram Premium user
     pub is_premium: Option<bool>,
     /// `true`, if this user added the bot to the attachment menu