optimise produceHashedIteratedS2k

```swift
    // this could be further optimised. Takes about 150ms per key
    // there tend to be two keys to decrypt in an armored key, so that makes is 300 ms
    // I suspect it could be optimised to 50ms per pass, or 100ms total
    func produceHashedIteratedS2k(_ algo: String, _ prefix: [UInt8], _ salt: [UInt8], _ passphrase: [UInt8], _ count: Int) -> [UInt8] {
```