This module implements a letter trie data structure. This is a linked set of
nodes representing a set of words. Starting from the root, each letter of an
included word is a daughter node of the trie. Hence, if a word is in the trie,
there will be a path from root to leaf for that word. If a word is not in the
trie, there will be no such path.

This structure allows for a relatively compact representation of a set of
words. This particular implementation allows each word to be stored alone or
with some associated data item.

Note that the remove() method does not prune nodes and thus a Trie can only
grow in size.