`difftime` is not represented in the S3 atomic vectors chart

[adamblake]

In 3.4 S3 atomic vectors, difftimes are described as one of the four important S3 vectors along with factor, Date, and POSIXct. In the image below the description only factor, Date, and POSIXct are represented.

Lines being referenced: https://github.com/hadley/adv-r/blob/a54903729dfa79fa79af5776218e6bc781e98c3d/Vectors.Rmd#L409-L413