rd_info.ParamSet suboptimal for function arguments

[sebffischer]

For parameter sets with functional parameters the parameter table contains the function source code as can be seen in the example below

library(mlr3extralearners)
learner = lrn("classif.C50")
learner$help()

^{Created on 2021-11-17 by the reprex package (v2.0.1)}

In this case it is not a big issue because the function is rather short but in general it might lead to issues.

[sebffischer]

Related issue: yihui/knitr#2075

[sebffischer]

Decision: Use substitute in p_uty to construct attribute repr that is then called by rd_info.ParamSet