Add64bit support

[chusitoo]

Hi @mourner and thanks a lot for the great libs that you share with the world!

I've been a user of Rbush for a few months now but only recently found out about Flatbush as a potential replacement due to its performance boost.

Although some C++ variants seem to already be available in the wild, I couldn't find anything that was up-to-date and that would exploit, arguably, one of its main strengths, of being able to recreate the exact state from a byte buffer on the other side, so I quickly put together my own interpretation of it in cpp.

This PR would be adding functionality for BigInt type (probably a niche scenario) as I'm planning to enable support for 64-bit integrals in my code as well (https://github.com/chusitoo/flatbush/tree/Support64Bit).

I also ported back some unit tests from my implementation that I found helpful, hoping to also improve test coverage on this side of things.

Cheers, Alex.