Value enum type safety

[stormpat]

Currently bindgen has some special privileges, not allowed in "normal" lisette code. Specially for the enum type, as it can have concrete values not allowed in user-land.

As an example from time.d.lis

// time.d.lis

pub enum Weekday: int {
  Friday = 5,
  Monday = 1,
  Saturday = 6,
  Sunday = 0,
  Thursday = 4,
  Tuesday = 2,
  Wednesday = 3,
}

// go/time/time.go

type Weekday int

const (
	Sunday Weekday = iota
	Monday
	Tuesday
	Wednesday
	Thursday
	Friday
	Saturday
)

This allows for good go -> lis -> go interop, but also misses the opportunity to be more type safe, meaning the type is (in the time.time case) still just an int, allowing for UB, as an example:

fn payday(day: time.Weekday) {
  fmt.Println("Pay the client on: ", day)
}

fn main() {
  payday(7) // BUG: Only 0-6 is valid as time.Weekday
}

Every go const + iota should be possible to infer at compile time, as only const expressions are valid, so i think there should be a high possibility to have lisette infer all valid uses, and not resort to a primitive like int on the lisette side. There are definitely edge cases here, but i think this could be something to explore further.

[ivov]

Good callout. Value enums were one of my earliest decisions when first building bindgen; this model where they're transparent with their underlying primitive needs revisiting. Turning this into an issue: #547

Edit: Sorted at #547 (comment)