This is a version of Time.Format that doesn't require allocation.

This is an updated version of 0af302f5
submitted by @bradfitz which was later rolled back.

Fixes #5192
Updates #5195

Change-Id: I4e6255bee1cf3914a6cc8d9d2a881cfeb273c08e
Reviewed-on: https://go-review.googlesource.com/1760Reviewed-by: Rob Pike <r@golang.org>