fix: simple ORM for one struct and foreign references work

Currently, all methods declared in the Georm trait are available.

If a struct has an ID pointing towards another entity, the user can
create a get method to get the entity pointed at from the database
too (local one-to-one relationship).

I still need to implement remote one-to-one relationships (one-to-one
relationships when the ID of the remote object is not available
locally).

I still need to also test and debug one-to-many relationships (ID of
the remote entiies not available locally) and many-to-many
relationships (declared in a dedicated table).

For now, IDs in all cases are simple types recognized by SQLx that are
not arrays. Options are only supported when explicitely specified for
one-to-one relationships.

tests/models.rs

@@ -0,0 +1,63 @@
+use georm::Georm;
+
+#[derive(Debug, sqlx::FromRow, Georm, PartialEq, Eq, Default)]
+#[georm(table = "biographies")]
+pub struct Biography {
+    #[georm(id)]
+    pub id: i32,
+    pub content: String,
+}
+
+#[derive(Debug, sqlx::FromRow, Georm, PartialEq, Eq, Default)]
+#[georm(table = "authors")]
+pub struct Author {
+    #[georm(id)]
+    pub id: i32,
+    pub name: String,
+    #[georm(relation = {entity = Biography, table = "biographies", name = "biography", nullable = true})]
+    pub biography_id: Option<i32>,
+}
+
+impl PartialOrd for Author {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.id.cmp(&other.id))
+    }
+}
+
+impl Ord for Author {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.id.cmp(&other.id)
+    }
+}
+
+#[derive(Debug, sqlx::FromRow, Georm, PartialEq, Eq, Default)]
+#[georm(table = "books")]
+pub struct Book {
+    #[georm(id)]
+    ident: i32,
+    title: String,
+    #[georm(relation = {entity = Author, table = "authors", name = "author"})]
+    author_id: i32,
+}
+
+impl PartialOrd for Book {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.ident.cmp(&other.ident))
+    }
+}
+
+impl Ord for Book {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.ident.cmp(&other.ident)
+    }
+}
+
+#[derive(Debug, sqlx::FromRow, Georm, PartialEq, Eq)]
+#[georm(table = "reviews")]
+pub struct Review {
+    #[georm(id)]
+    pub id: i32,
+    #[georm(relation = {entity = Book, table = "books", remote_id = "ident", name = "book"})]
+    pub book_id: i32,
+    pub review: String
+}