Add NIO Channel APIs for buffers

[jvz]

I was thinking it would be handy if I could either wrap an LBufferAPI into a SeekableByteChannel instance or similar. For instance, I may be handed a direct ByteBuffer from one API which I can't safely convert into an LBufferAPI instance without worrying about its internal cleaner, so being able to more obviously take advantage of bulk get/put APIs from the channel APIs would make a lot of sense. I see there is already a package-private implementation of a WritableByteChannel at least, so the idea is there.