internal/poll: rely on utf8.AppendRune

internal/poll: rely on utf8.AppendRune

This is both simpler and more performant.

Change-Id: I398de29230e578fb7caf1765df7b8fe84d4e8876
Reviewed-on: https://go-review.googlesource.com/c/go/+/412336
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>