Design for collections upgrades

[Brian Goetz]

 > I don't feel strongly that we must have an eager
> API, but I do feel like we can't have a streaming API that looks like
> an eager one.

I would agree with this position.  There are pluses and minuses for the 
eager version, but we can't make the default API either lazy or parallel 
- you have to ask for that, and it should be reflected in the (static) 
type system.