Add G2P as a single big commit.

        This change adds an experimental grapheme-to-phoneme converter
        that has been in development in its own branch for a while.

        The converter takes an *analysis string* and produces a
        phonetic string in IPA. Example use should something like:

            echo 'oku<V><neg><imp><2s>' | flookup -i g2p.fst

        All necessary mechanisms is in place (bugs aside), but lexicon
        needs a major update for non-default stress patterns and some
        of the borrowings to come out correctly.