context: use Go 1.8 type alias for CancelFunc and Context

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 <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
3 files changed
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