`->` field access not found on enums within state machine macro

[Jialin Li]

The -> field access works well on enums declared outside of the state machine macro, but produces "method arrow_b not found for this enum" for enums within the state machine macro.

state_machine!{ Example {
    fields {}

    pub enum Label
    {
        B{b: nat}
    }

    pub open spec fn test(t: Label) -> bool
        recommends t is B
    {
        t->b > 10
    }
}}