This looks like it is just moving some code from
time to runtime (and translating it to C), but the
runtime can do a better job managing the goroutines,
and it needs this functionality for its own maintenance
(for example, for the garbage collector to hand back
unused memory to the OS on a time delay).
Might as well have just one copy of the timer logic,
and runtime can't depend on time, so vice versa.

It also unifies Sleep, NewTicker, and NewTimer behind
one mechanism, so that there are no claims that one
is more efficient than another.  (For example, today
people recommend using time.After instead of time.Sleep
to avoid blocking an OS thread.)

Fixes #1644.
Fixes #1731.
Fixes #2190.

R=golang-dev, r, hectorchu, iant, iant, jsing, alex.brainman, dvyukov
CC=golang-dev
https://golang.org/cl/5334051