v1 release

[pellared]

I believe it would be beneficial to release go-cmp as v1.

Reasons:

1. The library is widely used, and I assume there’s no intention to introduce breaking changes while still in v0. If that’s the case, a v1 release would formally acknowledge its stability.
2. A stable v1 appears to be a prerequisite for resolving https://github.com/google/go-cmp/issues/338.
3. go-cmp is recommended by the official Go test comments guide. Given this endorsement, it would be ideal for the package to be versioned in a way that ensures long-term stability (i.e., avoiding breaking changes without a v2 release).
4. This will mitigate possible build failures if there if go-cmp is used by different Go test helper modules and they use different (incompatible) versions of go-cmp.