Fix encoding of large numbers

[jhermsmeier]

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So, this is a fun one I noticed yesterday: Number.toString() can not be relied upon to give you all digits of an integer:

// Number.toString()
> 340282366920938463463374607431768211456..toString()
'3.402823669209385e+38'

// Number.toPrecision() doesn't really help either
> 340282366920938463463374607431768211456..toPrecision(100)
'340282366920938463463374607431768211456.0000000000000000000000000000000000000000000000000000000000000'

// Neither does Number.toFixed()
> 340282366920938463463374607431768211456..toFixed(0)
'3.402823669209385e+38'

// Oh, look
> BigInt(340282366920938463463374607431768211456).toString()
'340282366920938463463374607431768211456'

While this is reasonably unlikely to happen in real-world usage as Number.toString() starts returning exponential notation at 2⁷⁰, it still could occur, and considering this might be run on other engines than V8 which might have different limits, this could potentially occur sooner.

Unfortunately the only fix I found for this is going through BigInt() – I guess we could hand-roll something, but BigInt() seems to be the reliable thing to go for here – what do you think?

[themasch]

I think this needs a new merge from master (with conflict solving in encode.js) and we need to remove the float test since this breaks with bigint.

[ThaUnknown]

superseeded by https://github.com/webtorrent/node-bencode/pull/150