makes the Context interface dependent only on standard packages, which
means types in other packages can implement this interface without
depending on go.net/context.

Remove the NewKey function and add examples showing how to use
unexported types to avoid key collisions.  This is the same model used
by http://www.gorillatoolkit.org/pkg/context, except we associate values
with a specific Context instead of storing them in a package-level map.

LGTM=crawshaw
R=golang-codereviews, crawshaw, dsymonds
CC=golang-codereviews, rsc
https://golang.org/cl/104480044