Flag the use of special columns when reading a fst file

[MarcusKlik]

A fst file might contain columns of non-standard types integer64, nanotime or ITime. To make sure these are processed and printed correctly, the corresponding packages need to be loaded (bit64, nanotime or data.table). The C++ core library (fstlib) returns column vectors with the appropriate class set, but the specific packages still need to be loaded.

For a large number of columns, scanning for these special column types using the class names can be very expensive.

To avoid running

sample_data <- data.frame(
  X = bit64::as.integer64(1:10),
  Y = 1:10)

# check for integer64 column
any(sapply(sample_data, inherits, "integer64"))
#> [1] TRUE

in read_fst() for all special types, the core library should return a vector of all column-types that were read. That vector can be used to selectively attach the correct packages, without the performance hit.