CancelFunc was the only part of the context package which didn't
forward nicely with the move from x/net/context to std context.

Use it for Context as well.

Change-Id: Ieff39b10b0783d55d0437c73923053297ed0ea4a
Reviewed-on: https://go-review.googlesource.com/32317
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>