refactor: use inherits, toString, explicit integer minor formatting

[m-muecke]

• use more explicit integers
• replace vapply() for mlr3misc functions
• use toString()
• minor whitespace formatting
• use inherits() for checking for try-error
• remove mlr3, mlr3misc, checkmate, and data.table explicit namespace since already importing
• add missing stats imports
• remove native pipe, since requiring R >= 4.1.0