proposal: extendable structs

[arjendevos]

We had a discussion on extending imported structs a while ago when building out the selective imports. However I'd imagine this is a complex feature and not sure if we want to have this.

I was thinking of making structs immutable to prevent extending, however I think we need to implement something better. I was looking into traits and I would like to hear your what you think about all this @weiwenhao.

[weiwenhao]

1. Allow duck-typed interface implementations like golang

2. Use something like rust impl to implement

impl int/string/float for T {

}

I'm not going to add impl yet until there's a greater need for it.