Request: Random.generate(hexCharacterCount:)

The `Random.generate(byteCount:)` is a useful way of generating random bytes, but in my own work I found I was mostly using it to generate random strings for encryption purposes. This meant every call to `Random.generate(byteCount:)` was followed by a call to `CryptoUtils.hexString(from:)` to get a Swift string rather than just bytes.

If you could implement a `Random.generate(hexCharacterCount:)` method that combined the two together, it would be most appreciated. If this were combined with #17, then this code:

if let randomBytes = try? Random.generate(byteCount: 64) {
randomString = CryptoUtils.hexString(from: randomBytes)
} else {
randomString = generateMyOwnRandomStringSomehow()
}

Would become simply this:

randomString = Random.generate(hexCharacterCount: 128)

(Note: There may well be a better name for this method.)