Put Message::thread behind a Box (#2658)

This trades a heap allocation for messages sent along with thread
creation for `Message`'s inline size dropping from 1176 bytes to 760
bytes,

Author: GnomedDev

src/model/channel/message.rs

@@ -112,7 +112,7 @@ pub struct Message {
     /// [`Interaction`]: crate::model::application::Interaction
     pub interaction: Option<Box<MessageInteraction>>,
     /// The thread that was started from this message, includes thread member object.
-    pub thread: Option<GuildChannel>,
+    pub thread: Option<Box<GuildChannel>>,
     /// The components of this message
     #[serde(default)]
     pub components: Vec<ActionRow>,

src/model/event.rs

@@ -534,7 +534,7 @@ pub struct MessageUpdateEvent {
     #[serde(default, deserialize_with = "deserialize_some")]
     pub interaction: Option<Option<Box<MessageInteraction>>>,
     #[serde(default, deserialize_with = "deserialize_some")]
-    pub thread: Option<Option<GuildChannel>>,
+    pub thread: Option<Option<Box<GuildChannel>>>,
     pub components: Option<Vec<ActionRow>>,
     pub sticker_items: Option<Vec<StickerItem>>,
     pub position: Option<Option<u64>>,