Implement NEP 2

[kylef]

This is a reference implementation of NEP 2.

[danneu]

What do you think about changing next to return byte arrays instead of single bytes?

i.e. change this

protocol PayloadType {
  mutating func next() -> UInt8?
}

to

protocol PayloadType {
  mutating func next() -> [UInt8]?
}

Reasoning:

For all responses except for streams, you already have the payload in memory, in which case you have to regress from what was a straight socket dump to a tight loop just for conformance.

Also, (so far in my limited experience with Swift) it seems that you usually end up with byte arrays when working with APIs and system calls to read data. For example, when I was extending NSInputStream to conform to PayloadType, I was getting [UInt8] chunks from the stream at which point emitting individual bytes felt like an unnecessary intermediate step.

Finally, I'll sheepishly admit that I had trouble making the tight loop performant. Maybe it's me (most likely), maybe it's the socket abstraction I am using, but I definitely had a big performance regression compared to just flushing byte arrays to the socket.

What do you think?

[kylef]

@danneu I agree and I've added that change.