Consider adding `#[must_use]` to `#[table]` rows in Rust modules

Upsides

This would catch anywhere the user updates a row struct but forgets to commit it. This is likely to be particularly helpful to new users who don't understand the data model, but it would help experienced users as well.

E.g.

fn update_username(ctx: &ReducerContext, id: UID, name: String) {
   let mut user = ctx.db.user().id().find(id);
   user.name = name;
   // whoops! this does nothing!!
}

Downsides

You have to explicitly discard rows you don't need, e.g.

fn update_username(ctx: &ReducerContext, id: UID, name: String) {
   let mut user = ctx.db.user().id().find(id);
   user.name = name;
   // now you need this `let _ =` to avoid a warning, because the `update` method returns the updated row.
   let _ = ctx.db.user().id().update(user);
}

This makes using Row structs as plain-old-structs slightly less convenient. I still think it's worth it.

Start by locating how #[table] row types are defined or generated in Rust modules and how row updates are represented. Assess whether #[must_use] can cover forgotten row commits without making intentional discarded results impractical. Done means the behavior is implemented consistently and the examples in the issue’s update and explicit-discard cases are covered.