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.

migrations/20250126153330_simple-struct-tests.up.sql

@@ -0,0 +1,38 @@
+CREATE TABLE biographies (
+    id SERIAL PRIMARY KEY,
+    content TEXT NOT NULL
+);
+
+CREATE TABLE authors (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100) NOT NULL,
+    biography_id INT,
+    FOREIGN KEY (biography_id) REFERENCES biographies(id)
+);
+
+CREATE TABLE books (
+    ident SERIAL PRIMARY KEY,
+    title VARCHAR(100) NOT NULL,
+    author_id INT NOT NULL,
+    FOREIGN KEY (author_id) REFERENCES authors(id) ON DELETE CASCADE
+);
+
+CREATE TABLE reviews (
+    id SERIAL PRIMARY KEY,
+    book_id INT NOT NULL,
+    review TEXT NOT NULL,
+    FOREIGN KEY (book_id) REFERENCES books(ident) ON DELETE CASCADE
+);
+
+CREATE TABLE genres (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100) NOT NULL
+);
+
+CREATE TABLE book_genres (
+    book_id INT NOT NULL,
+    genre_id INT NOT NULL,
+    PRIMARY KEY (book_id, genre_id),
+    FOREIGN KEY (book_id) REFERENCES books(ident) ON DELETE CASCADE,
+    FOREIGN KEY (genre_id) REFERENCES genres(id) ON DELETE CASCADE
+);