time: add Duration.Truncate

[cespare]

See @rsc's comment at #18991 (comment).

I was thinking of adding:

// Truncate returns the result of rounding d towards zero
// to a multiople of td.
func (d Duration) Truncate(td Duration) Duration

I'll send a CL but I want to make sure this looks good.

[cespare]

Do we need Round as well, then?

[rogpeppe]

Yeah, if we have Truncate, I reckon we should have Round too.

[rsc]

Yes, Duration and Round together please. Let's call the parameter some single letter, like m (as in "multiple of m").

[gopherbot]

CL https://golang.org/cl/36615 mentions this issue.