The as_dt could turn any data frame to data.table class. If the data is
not a data frame, return error.
The in_dt function creates a virtual environment in data.table, it could be
piped well because it still follows the principals of tidyfst, which are: (1) Never
use in place replacement and (2) Always recieves a data frame (data.frame/tibble/data.table)
and returns a data.table. Therefore, the in place functions like := will still
return the results.