time: add Since, which returns the time elapsed since some past time t.

R=rsc, r
CC=golang-dev
https://golang.org/cl/5532088

src/pkg/time/time.go

@@ -566,6 +566,12 @@ func (t Time) Sub(u Time) Duration {
 	return Duration(t.sec-u.sec)*Second + Duration(t.nsec-u.nsec)
 }
 
+// Since returns the time elapsed since t.
+// It is shorthand for time.Now().Sub(t).
+func Since(t Time) Duration {
+	return Now().Sub(t)
+}
+
 // AddDate returns the time corresponding to adding the
 // given number of years, months, and days to t.
 // For example, AddDate(-1, 2, 3) applied to January 1, 2011