Optimise word lists for common operations

[allenap]

This follows on from ideas in #76:

... space could be saved if words were not stored as a static newline-separated string but as something like (&'static [[u8; 1]], &'static [[u8; 2]], &'static [[u8; 3]], ...), i.e. lists of words indexed by length (even more efficient than [&'static str] because we don't need one reference per word). I hope such a list can be built at compile-time from the newline-separated files.

There are at least a couple of common things that the petname command-line tool lets you do that might benefit from preprocessing the word lists before they're compiled in, i.e. alliteration, and word length limits:

    -a, --alliterate                  Generate names where each word begins with the same letter
    -A, --alliterate-with <LETTER>    Generate names where each word begins with the given letter

    -l, --letters <LETTERS>           Maximum number of letters in each word; 0 for unlimited [default: 0]

Separately, I am thinking about changing the -l, --letters <LETTERS> option to take a range, e.g. 3-8. That might have a bearing on how to preprocess the default word lists.

[allenap]

#60 is also relevant here. It may be useful to have lists collated by first letter.