ByteBuffer: small & medium sized string copies over 10% faster (#640)
ByteBuffer: small & medium sized string copies over 10% faster (#640)

Motivation:

urbp[start ..< end] is quite a bit faster than when done in two
operations urbp.dropFirst(start).prefix(length). This improves small (4
bytes) & medium sized (24 bytes) String copies by over 10%. Good for
HTTP headers etc.

Modifications:

replace urbp.dropFirst(start).prefix(length) by urbp[start ..< start +
length]

Result:

small & medium sized String copies faster
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio