Implement xfirst and xlast to control for directional crossing of the DST gap.

[vspinu]

library(timechange)
tt <- as.POSIXct(c("2014-11-02 00:15:00", "2014-11-02 02:15:00"), tz = "America/New_York")
time_add(tt, hours = c(1, -1), roll_dst = "pre")
#> [1] "2014-11-02 01:15:00 EDT" "2014-11-02 01:15:00 EDT"
time_add(tt, hours = c(1, -1), roll_dst = "post")
#> [1] "2014-11-02 01:15:00 EST" "2014-11-02 01:15:00 EST"
time_add(tt, hours = c(1, -1), roll_dst = "xfirst")
#> [1] "2014-11-02 01:15:00 EDT" "2014-11-02 01:15:00 EST"
time_add(tt, hours = c(1, -1), roll_dst = "xlast")
#> [1] "2014-11-02 01:15:00 EST" "2014-11-02 01:15:00 EDT"

Lubridate's issue tidyverse/lubridate#1088