* Performance improvements.

* Removed `Param` objects. `ParamSet` now uses a `data.table` internally; individual parameters are more like `Domain` objects now. `ParamSets` should be constructed using the `ps()` shorthand and `Domain` objects. This entails the following major changes: * `ParamSet` now supports `extra_trafo` natively; it behaves like `.extra_trafo` of the `ps()` call. * `ParamSet` has `$constraint` * `ParamSet` objects are now less mutable. The only properties that can be changed are `values`, `tags`, `deps`, `constraint` and `extra_trafo`. * `ParamSet$is_bounded` is a vector with an entry for each parameter. Use `$all_bounded` for the previous behavior. * `Condition` objects are now S3 objects and can be constructed with `CondEqual()` and `CondAnyOf()`, instead of `CondXyz$new()`. (It is recommended to use the `Domain` interface for conditions, which has not changed) * `ParamSet` has new fields `$is_logscale`, `$has_trafo_param` (per-param), and `$has_trafo_param` (scalar for the whole set). * Added a vignette which was previously a chapter in the `mlr3book`

* Minor bug fixes.

* feat: The function `generate_design_sobol()` generates a space-filling Sobol sequence design. * refactor: `$set_values` returns the parameter set invisible.