I realized that the C version of this thing does multiple things
in the same function: it loads the bigrams, it iterates through
the database, and it compares the things found in the database
to the prepared pattern. It seems to me, therefore, that we're
better off with an instance that loads the bigrams, then closes
the database immediately.
Later, the client can ask for one of two iterators: one that either
returns each entry in sequence, or one that returns each entry in sequence
that matches the pattern passed in.
