[feature request] add error-awared functions like: `makei_error`, `map_error`

[tonyfettes]

It's often the case when I want to construct an Array[T] or FixedArray[T] from a function that can raise error. For example, I want to construct a Tensor from a Json value:

fn main {
  match json {
    Json::Number(x) => Tensor::new(x)
    Json::Array(xs) => {
      let value : FixedArray[Tensor] = FixedArray::makei(xs.length(), fn(i) {
        Tensor::from_json!(xs[i], @json.add_index(path, i))
      })
      Tensor::stack(value)
    }
    _ => raise @json.JsonDecodeError((path, "Expected an array"))
  }
}

For now it is not possible to achieve this, since FixedArray::makei require the callback to be error-free.

[htoooth]

@tonyfettes Maybe the functions of all core module have to add the error feature. But i wish you do not use error feature in you code. Because this feature influences all related code if you use it. Just like now scene you face.

[bobzhang]

let value : FixedArray[Tensor] = FixedArray::makei(xs.length(), fn(i) {
        Tensor::from_json!(xs[i], @json.add_index(path, i))
      })
Tensor::stack(value)

Note you work around this using for .. in, I am hestitant to add such API since eventually want our map to propagate callbacks with Error something like this:

map[A](self : Array[A], f : (A) ->B!_) -> Array[B]!_ // when call back has error, result has error