doc: update go1.8.txt using new tool

With help of a new interactive commit classifier tool (tool location
TBD, likely x/build/cmd/writenotes), classify all commits from go1.7
up to 56d35d42.

We can selectively cull this list later. When in doubt, I erred on the
side of inclusion for now.

Change-Id: I458945004e1b1a148fb2f294b454a390ef4f92c2
Reviewed-on: https://go-review.googlesource.com/30696Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>