RLTK

An attempt to manually port some of nltk to rust.

Currently in it's infancy:

• rltk::lm::preprocessing::pad_both_ends(["a","b","c"], 2) -> "<s>", "a", "b", "c", "</s>"]
• rltk::util::pad_sequence == same as above with customisation
• rltk::util::pad_sequence_left == same
• rltk::util::pad_sequence_right == same
• rltk::util::ngrams(["a","b","c"],2) -> "a"], ["b"], ["b"], ["c"
• rltk::util::bigrams(["a","b","c"]) == ngrams(..., 2)
• rltk::util::trigrams(["a","b","c"]) == ngrams(..., 3)