proposal: add package version support to Go toolchain

It is long past time to add versions to the working vocabulary of both Go developers and our tools.
The linked proposal describes a way to do that. See especially the Rationale section for a discussion of alternatives.

This GitHub issue is for discussion about the substance of the proposal.

Other references:

• https://research.swtch.com/vgo, the detailed designs
• A Tour of Versioned Go, a walkthrough of what it's like to use
• go get golang.org/x/vgo, the prototype implementation
• https://golang.org/wiki/vgo, links to related posts, videos, etc
• vgo milestone on issue tracker