Replace Long.js conversion of i64 with JS BigNum support

[danleh]

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)

[doehyunbaek]

I think this commit https://github.com/jakobgetz/wasabi/commit/280947b94db2a19072575f100a9387904661d45d is quite minimal and easy to review. If you don't mind I'll make a pull request cherry picking this commit.

[danleh]

That commit looks great, if you could make a PR out of it and it passes tests, we can merge it as-is.

[doehyunbaek]

I'll work on this in a few weeks. Forgot in a hurry!