Support BigInt

[betamos]

The builtin BigInt type is supported in many browsers and in NodeJS (such as the fs.stat API).

Current behavior is particularly spurious and can cause bugs: Long.fromValue(55n) results in 0

Please consider adding support for BigInt or if you'd rather accept contributions I'd be happy to take a look.

[jimmywarting]

I maybe think it should rather be suggested that BigInt is used instead of long?

[martinheidegger]

In longfn I implemented BigInt for the from/toBytes methods:

toBigInt(fromBytesLE(bytes,...))

But independently of that, I think it is a good idea to have a int64 implementation handy when needing to be computationally compatible to other implementations. With BigInt buffer overflows don't exist...