use serde deserializer functions to remove duplicated structs, remove TryFrom impls, replace Language enum with wrapper around isolang Language type

src/metadata.rs

@@ -2,8 +2,9 @@
 
 use serde::Serialize;
 use crate::response_deserializer::{
-    Chapter, ContentRating, Language, Manga, PublicationDemographic, State, Status, Titles,
+    Chapter, ContentRating, Manga, PublicationDemographic, State, Status, Titles,
 };
+use crate::language::Language;
 use chrono::{DateTime, FixedOffset};
 
 #[derive(Serialize)]
@@ -26,7 +27,7 @@ pub struct Metadata {
 struct ChapterMetadata {
     chapter: f32,
     volume: f32,
-    title: String,
+    title: Option<String>,
     pages: u32,
 }