Update English checker

[bee-san]

This updates the English checker to work on multiple words. Previously it only worked on 1 word.

There are some notable changes in this PR:

• Using Cargo-Nextest instead of the regular Cargo test, which runs
  tests faster

There are 2 bugs in the English checker:

 // TODO: I think the below function iterates through each dictionary in turn.
            // Which means it'll try English.txt, then rockyou.txt etc
            // This is inefficient and makes it harder to compute what dictionary the word came from.
            // We should probably just use a single dictionary and assign the filenames to the values in the dictionary.
            // Like {"hello": "English.txt"} etc.
            // If we're using muiltiple dictionaries we may also have duplicated words which is inefficient.
            if storage::DICTIONARIES.iter().find(|(_, words)| words.contains(word)).is_some() {
                trace!("Found word {} in English", word);
                words_found += 1.0;
            }

And:

 // TODO: There is a bug here. We are comparing how many words found to how many characters there are in bytes.
            // This means the numbers don't exactly match up. There may be some cases where this will break.
            // We are also typecasting to f64 instead of usize, which costs CPU cycles.
            if words_found / input.len() as f64 > PLAINTEXT_DETECTION_PERCENTAGE {
                debug!("Found {} words in {}", words_found, input);
                debug!("Returning from English chekcer successfully with {}", input);
                plaintext_found = true;
                break;
            }

I will fix these in a future PR. This PR is designed to just let
Skeletal work on a Caesar cipher :)

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