Since Go 1.7 started including the context package in the core
distribution, this package has been forwarding to the core package's
implementation to the best of its ability. Since type aliases are due
to be included in the Go 1.9 release, use them to forward more
completely.

Change-Id: I97b92cd34b3f4426f565bb3d115f119d3c83a1dd
Reviewed-on: https://go-review.googlesource.com/49020
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>