generator: Apply must_use attributes to all struct setter functions

All structs are marked as `Copy`, and builders move `self` for a
convenient builder pattern directly on `default()` (using `&mut`
requires first storing the builder on the heap).  This however leads
to builder functions accidentally moving a copy of `self`, mutating it,
and dropping the result directly when the caller did not consume the
returned value in ad-hoc field updates, in turn leaving the author
confused why their code compiles without warnings while the struct was
not updated.

Annotating all builder functions with `#[must_use]` should at least give
them an idea why.