These classes are used to define callback behaviors, and are based
on readr's readr::callback functions.
The callbacks HipChunkCallback, HipListCallback and
HipSideEffectChunkCallback should be identical to their readr
counterparts, but have been copied into hipread to ensure that they
work even if readr changes.
The callback HipDataFrameCallback is similar to
readr::DataFrameCallback() except that it uses dplyr::bind_rows()
instead of rbind() so that it is faster.