Replace Long.js conversion of i64 with JS BigNum support

See https://github.com/danleh/wasabi/issues/31#issuecomment-786532720 Copied from there: remove Long.js dependency completely and use JavaScript BigNums instead, see https://github.com/WebAssembly/proposals/blob/master/finished-proposals.md and https://github.com/WebAssembly/JS-BigInt-integration * this wasn't possible back in 2018 because the proposal wasn't finished yet * [ ] check browser and Node.js compatibility of JS BigNums * [ ] change low-level hook code to use BigNums instead of Long.js (should be quite easy/mechanical)