Support 64 bit integers in tasks

[mllg]

I.e., via package bit64. data.table already supports it nicely.

[pat-s]

Any examples on its effect?

[mllg]

Is needed for row ids if you have more than .Machine$integer.max rows. Alternatively also allow doubles, but this is rather ugly.

[gsenseless]

Any examples on its effect?

require(bit64) task = TaskRegr$new("mc", backend = data.table(dd = bit64::as.integer64(34534543), bb = 0), target = "bb") #Error in super$initialize(id = id, task_type = task_type, backend = backend) : #Assertion on 'feature types' failed: Must be a subset of #{'logical','integer','numeric','character','factor','ordered','POSIXct'}, but is {'integer','numeric','integer64'}.