This patch adds the find_word_ranges method to the WordSeparator struct
that makes it possible to find words within a string without using
textwrap’s Word type.  This is especially useful when using custom types
for strings.

This patch adds the word_splitters::Fragments struct that yields the
fragments for a word.  This makes it easier to reason over the lifetimes
of the generated iterator and allows us to make it generic over the word
type.

This patch introduces the new word_splitters::Splittable trait and makes
word_splitters::Fragments generic over that trait.  This allows library
users to use their own fragment types and not only core::Word.

Configure required features for style example