64 bit integer handling

[dbrgn]

JS only has 53 bit precision integers.

To be able to handle 64 bit precision integers, use the int64 option for msgpack-lite and require int64-buffer as peer dependency.

https://github.com/kawanet/msgpack-lite
https://github.com/kawanet/int64-buffer

[dbrgn]

Ah, actually it's a msgpack-lite dependency, so we don't need a peer dependency 🙂