Skeletal start to reaction roles

src/reaction_roles.rs

+
+#[derive(Debug, Clone)]
+struct ReactionMapping {
+    mapping: HashMap<serenity::model::id::EmojiId, serenity::model::id::RoleId>,
+}
+
+lazy_static! {
+    static ref REACTIONS_CACHE: Mutex<HashMap<serenity::model::id::MessageId, ReactionMapping>> =
+        Mutex::new(HashMap::new());
+}