time: add AfterFunc to call a function after a given duration.

time: add AfterFunc to call a function after a given duration.
The After code is trivially generalisable to provide support
for this, and it is possible to use AfterFunc to do
things that After cannot, such as waiting
for many events at varied times without an overhead
of one goroutine per event.

R=rsc, r
CC=golang-dev
https://golang.org/cl/3905041