We should support incremental indexing of large corpuses.  We can already save a corpus
to an io.Writer and load a corpus from an io.Reader, but once we have a large, saved
Corpus, there is no way to update just part of it.

I think we should add a method on Index, like:

func (x *Index) Add(o *Index) *Index

That returns the merge of x + o (with any redundant text in the bytes.Buffer/Fileset
removed and compacted)

Then incremental indexing just involves loading the last huge index from disk, building
small indexes of subtrees, merging them in, writing to disk, etc.

Perhaps the Index.Add method only adds and is quick, but we could also have a Compact
method to return a compacted index (a slower operation), so the common case is a bunch
of adds, followed by a less-frequent compact + write to disk.