mlr3 0.6.0

• Compact in-memory representation of R6 objects to save space when
  saving objects via saveRDS() or serialize().
• Objects in containers like ResampleResult or BenchmarkResult are now
  de-duplicated for an optimized serialization.
• Fixed data set breast_cancer: all factor features are now
  correctly stored as ordered factors.
• Added a new utility function convert_task().