Fix DoS via getNthKey

[JeremyMoeglich]

Previously getNthKey might be called many times each time iterating through a full user provided collection.
By providing a large collection and a large referentialEqualities array you could cause quadratic compute.
For a 1mb payload (a common webserver limit) this can block the event loop for 20 - 40 seconds

Benchmarks (run on a Ryzen 7 5700X, Arch Linux)
Previous

NodeJS:
baseline: 37.97 ms
small attack: 439.44 ms
double inputs: 1695.84 ms (3.85x)
optimized: 30912.67 ms

Bun:
baseline: 28.13 ms
small attack: 524.92 ms
double inputs: 2063.38 ms (3.93x)
optimized: 45527.36 ms

After this change:

NodeJS:
baseline: 62.29 ms
small attack: 14.02 ms
double inputs: 17.99 ms (1.28x)
optimized: 101.53 ms

Bun:
baseline: 52.97 ms
small attack: 17.19 ms
double inputs: 27.44 ms (1.60x)
optimized: 94.58 ms

[Skn0tt]

Hi Jeremy, thanks for your contribution. I don‘t like how this minor fix significantly increases complexity. Let‘s discuss the problem in an issue and see what are possible alternatives.

[JeremyMoeglich]

This PR also seems to have a regression I'll have to fix if we want to take this approach over yours.
Though I would still prefer to take your approach as that reduces code quite a bit.

import superjson from 'superjson';

const set = new Set();
const root = { back: set };

set.add(null);
set.add(root);

try {
  const json = superjson.stringify(root);
  const _ = superjson.parse(json);
  console.log("✅ Success");
} catch (e) {
  console.error("❌ Crash detected:", e.message);
}