fix: wrap go interface returns as option

[ivov]

Go functions and methods returning an interface now correctly bind as Option<T>, reflecting the fact that any Go interface value can be nil. Previously Lisette claimed the result was always present, so a caller could deref into a runtime panic with no protection from the type checker.

import "go:image"

fn first_pixel(img: image.Image) -> string {
  match img.At(0, 0) {
    Some(c) => "got color",
    None => "no pixel",
  }
}

The same change applies to function returns where the body has a return nil path, such as container/ring.New(0) and math/big.Int.ModInverse.

For well-known constructors that never return nil in practice but whose multi-path bodies cannot be statically narrowed, explicit allowlist entries in bindgen.stdlib.json keep the original non-Option binding.

Close #438

[github-actions]

Released in lisette-v0.2.8