Support annoy index loading from binary index data

[akg9023]

Many times, I have felt a need to save the index file in some database and load it directly from there. Since, the file is already in memory, it makes less sense to store the index data in a temporary file and then mmap file to AnnoyIndex.

I feel it would be great if we can have an interface for supporting load() from binary index data as well.

The other option is to use some kind of filestore, but keeping a filestore just for indexes becomes an overhead.
I would love to hear any other options we could use/include.