Large unsigned integers should be decoded to a BigInt rather than a string

[oschwald]

Currently, the decoder converts unsigned integers that are more than 6 bytes long to strings. They would be easier to use correctly if they were left as BigInt values.

I am raising this as an issue rather than immediately making a PR as this would technically be a breaking change.

[runk]

It makes total sense to use BigInt.

I'm fairly sure that at the time of writing this lib (was core part of the maxmind module back then), there was no BigInt in javascript.

[oschwald]

Done in #231