There's no need for distinct hmap and hiter types for each map.

Shaves 9kB off cmd/go binary size.

Change-Id: I7bc3b2d8ec82e7fcd78c1cb17733ebd8b615990a
Reviewed-on: https://go-review.googlesource.com/c/go/+/521615


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>