[Feature Request] Import from /Export to Array Buffer, like Flatbush

[hydrosquall]

Hi,

Thanks for the work on this very helpful library!

I landed here from a comment in the companion library, which has methods that support serializing the index to disk (via the data property on an index instance), and loading that serialized result back conveniently (via Flatbush.from). It would be really convenient to have those same methods available for this library too.

https://github.com/mourner/flatbush/issues/19#issuecomment-429104887

[mourner]

Yes, I'd like to bring it on par with Flatbush in this regard — will hopefully take on this in the next few weeks!