speed up getBytes (#761)
speed up getBytes (#761)

Motivation:

Due to https://bugs.swift.org/browse/SR-9604, ByteBuffer.getBytes
seriously regressed (factor 10x) since NIO 1.11 . This works around it
until the issue is fixed.

Modifications:

bind the memory to UInt8s to get an UnsafeBufferPointer<UInt8> rather
than an UnsafeRawBufferPointer.

Result:

- getBytes 10x faster
- make @John-Connolly happy, thanks for reporting!
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio