Support hms objects in the `secs` arg

[moodymudskipper]

The round_hms() and trunk_hms() functions have a secs argument used to round or truncate to the closest multiple.
This expects an atomic number, but it would be great if we could provide a hms object to this argument, to easily round to the minute or hour.
In fact it would come for free if a pesky difftime method didn't get in the way.

library(hms)
t <- hms(100 * 1:5)

t
#> 00:01:40
#> 00:03:20
#> 00:05:00
#> 00:06:40
#> 00:08:20

round_hms(t, hms(minutes = 1))
#> Error in `/.difftime`(as.numeric(x), secs): second argument of / cannot be a "difftime" object

round_hms(t, as.numeric(hms(minutes = 1)))
#> 00:02:00
#> 00:03:00
#> 00:05:00
#> 00:07:00
#> 00:08:00

Should we vctrs::vec_cast(secs, numeric()) and document the feature ?

Defining /.hms would solve this too, and I see this is in progress in #120