Generics details 8: interface default and final members

[josh11b]

Allowing interfaces to define default values for its associated entities. This:

• Helps with evolution by reducing the changes needed to add new members to an interface.
• Reduces boilerplate when some value is more common than others.
• Addresses the gap between the minimum necessary for a type to provide the desired functionality of an interface and the breadth of API that user's desire.

As an alternative, final values can be provided instead, which can't be overridden, but are more predictable for users and may avoid dynamic dispatch overhead in some cases.

Example:

// Interface parameter has a default of `Self`
interface Add(Right:! Type = Self) {
  // `AddWith` *always* equals `Right`
  final let AddWith:! Type = Right;
  // `Result` has a default of `Self`
  let Result:! Type = Self;
  fn DoAdd[me: Self](right: Right) -> Result;
}

impl String as Add() {
  // Right == AddWith == Result == Self == String
  fn DoAdd[me: Self](right: Self) -> Self;
}

[zygoloid]

This looks great. I think final defaults may need a little more discussion before I'd consider us to have consensus here, but the rest looks good to go to me.

[josh11b]

I added some text and made some small reorganization based on recent discussion:

7c9c95d

[zygoloid]

Looks good to me. Soliciting more opinions before approving.

[chandlerc]

Generally makes sense. Minor comment about the design doc, but not at all blocking or even really interesting. Just a. doc improvement comment.

[chandlerc]

(now actually posting the pending comment)

[chandlerc]

Super minor (and fine for a follow-up): there isn't much motivation here in the design for this feature compared to others. The motivation in the proposal makes sense to me, but maybe three is a way to briefly summarize that so it doesn't read like an omission here?

[josh11b]

Done in 620d6de