Leanify package, add resamplings to dictionary

[sebffischer]

Summary of the PR:

• leanifying a package moves the code of the methods into the mlr3resampling namespace. The methods of the R6 instances then just call into these functions. In the past we often had problems with very large object sizes and this mitigates this.
• the resamplings are added to the mlr3::mlr_resamplings dictionary. This allows to construct resamplings using rsmp("<resampling-id>"), which we recommend over the Resampling$new() syntax
• removed one unnecessary call to task$data()

[tdhock]

hi thanks for the PR
can you please add package:: prefix to leanify_package() to make it clear what pacakge it comes from? (I don't know where, is it mlr3?)