Add safety checks of `.f`

[TimTeaFan]

It is possible that the function in .f uses the same variable as specified in idx which defaults to i. There should be a check in place, checking this , raising an error with the advise to set idx to a variable that is not used in .f.

map(1:4,  function(x) {
  i <- 5
  x + i
  }) %>% as_loop()

Need to solve #6 before solving this issue.

There also be other safety checks in place for all possible inputs like .sel .inp1 etc.