refact(rivetc.codegen): don't alloc tagged enum's struct values

[StunxFS]

With this PR, this snippet:

enum Tagged {
    StructVariant {
       field: int;
    }
}

func main() {
    t := Tagged.StructVariant(field: 1);
}

Will no longer require a heap allocation.

Mutability is also forced more, not only are the fields verified to be mutable, but also the objects that have said fields.