Add node crypto.randomBytes as engine about random-js

[AccaliaDeElementia]

I implemented this in #18 as part of the browserCrypto engine based off of @zzxz's investigation. I went with 32 bit signed integers for the randomly generated values as that is what the existing browserCrypto function expected.

After investigating the implementation differences between webbrowsers crypto.getRandomValues and nodejs's crypto.randomBytes i decided that there was no need to cache a pregenerated entropic array of results. NodeJS handles generation of the randomness differently.

I would have liked to use the asynchronous version of the randomBytes call, but that would have required a massive refactor of the entire module and i felt was not worth the effort at this time.

[ckknight]

Absolutely worth looking into.

[eozu]

Here's a toy example to get a 32-bit unsigned int:

var crypto = require('crypto')

// Synchronous version.
var buffer = crypto.randomBytes(4) // => <Buffer 12 26 7a d4>

// Read a 32-bit unsigned int from the buffer.
var number = buffer.readUInt32BE()

// ...do unbiased things with the number :)

We could, like browserCrypto, fill a larger buffer and read multiple values.

Buffer allows us to read direct up to 48-bits of accuracy using readUIntBE or readUIntLE. Without going too far (as I'm not a crypto/random expert), getting into compliance with random-js 53-bits might require a bit more work.

Interestingly enough, we can simplify the random-js API to nativeMath, crypto, and mt19937() (while still supporting browserCrypto). Users of random-js could produce code that runs on either client or server without code changes.

[ckknight]

This is available with v2.0.0-rc2.

const { nodeCrypto } = require('random-js');

nodeCrypto.next(); // uses require('crypto').randomBytes under-the-hood