Version that uses BigInt

[Daniel15]

It would be nice to get a version of this xxHash64 library that uses BigInt rather than the userland UInt64 implementation, in order to improve performance in environments that support BigInt. What do you think?

[pierrec]

Totally agree. Want to send a pull request?

[Daniel15]

Sure, I can try to take a look. Will see if I can figure out a way to have a version that uses native bigints and another version that uses a userland implementation, without duplicating much code.