Speed up using trigrams

[msuchane]

Before comparing the file content using the Levenshtein or Jaro distance, first compare the two files using word-level trigrams to get the general sense of their similarity. Then, use the distance metric only on files that are relatively similar by trigrams.

Resources:

• https://en.wikipedia.org/wiki/Trigram_search
• https://en.wikipedia.org/wiki/Trigram

[msuchane]

With version 0.5.0, the tool now pre-selects using character-level trigrams. As a result, the search is about 10 times faster.

Word-level trigrams could produce more accurate results and might even be faster, but no library can currently calculate them.

I'm leaving this open to consider word-level trigrams in the future.

[msuchane]

The slice::windows method would be quite useful when implementing word trigrams.