Implement has_X as an analogue to remove_X

[ErdaradunGaztea]

Especially useful for dplyr::filter usage. Right now it's necessary to write the code like that:

sq_tibble %>%
  mutate(sq = remove_ambiguous(sq)) %>%
  filter(length(sq) > 0)

We'd rather have it like that:

sq_tibble %>%
  filter(!has_ambiguous(sq))

Another point is that the latter preserves sequences of length 0 if such were present in the original tibble.