Some properties on the builder are shared with the index, so
we can save a bit of serialization space by not serializing
the exact same data twice

lunr.Builder.fieldTermFrequencies and lunr.Builder.fieldLengths
have the exact same keys, and are both keyed by field refs.  It
saves us some space when serializing a mutable builder to save
the list of field refs once, along with the values in these two
objects, and recombine them into objects at load time

Otherwise, if you're performing multiple changes to the index in-between
queries, you end up doing a lot of redundant work