io: adopt Discard, NopCloser, ReadAll from io/ioutil
As proposed and approved in #40025, Discard, NopCloser, and ReadAll
do not really fit into io/ioutil, which exists mainly to hold things that
would cause an import cycle if implemented in io itself, which is to say
things that import "os".
These three do not import "os" - they are generic io helpers like
many of the things in io itself, so it makes sense for them to be there.
Fixes #40025.
Change-Id: I77f47e9b2a72839edf7446997936631980047b67
Reviewed-on: https://go-review.googlesource.com/c/go/+/263141
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org>