Continued data.table support

[SebKrantz]

collapse supports data.table (it has become a partner package), which includes the ability to output valid data.table's after collapse operations. Currently, data.table's are implemented as overallocated lists, requiring collapse to make an additional shallow copy after manipulating the data.table and re-create the ".internal.selfref" attribute. But this frunctionality is non-API compliant, and the team is planning an ALTREP solution (https://github.com/Rdatatable/data.table/issues/6180#). This will affect collapse's handling of data.table's.