hash/maphash: finalize API design

[rsc]

hash/maphash's API has one TODO in a public doc comment (MakeSeed) and also various non-idiomatic duplication, like both AddBytes and Write methods that do the same thing.

I sent CL 205069 to fix up the API before we issue it in the Go 1.14 release.

Getting the API finalized obviously blocks its inclusion in the release.

[gopherbot]

Change https://golang.org/cl/205069 mentions this issue: hash/maphash: revise API to be more idiomatic