mlr3fselect 0.4.0

• Compact in-memory representation of R6 objects to save space when saving mlr3
  objects via saveRDS(), serialize() etc.
• FSelectorRFE supports fraction of features to retain in each iteration
  (feature_fraction), number of features to remove in each iteration
  (feature_number) and vector of number of features to retain in each
  iteration (subset_sizes).
• AutoFSelect is renamed to AutoFSelector.
• To retrieve the inner feature selection results in nested resampling,
  as.data.table(rr)$learner[[1]]$fselect_result must be used now.
• Option to control store_benchmark_result, store_models and check_values
  in AutoFSelector. store_fselect_instance must be set as a parameter during
  initialization.
• Adds FSelectorGeneticSearch.
• Fixes check_values flag in FSelectInstanceSingleCrit and
  FSelectInstanceMultiCrit.
• Removed dependency on orphaned package bibtex.
• PipeOpSelect is internally used for task subsetting.