(File)Channel support in Data I/O

Data I/O classes like provide methods to wrap existing objects, streams or byte buffers, and leverage uniformed consistent Data I/O APIs on top of them. It would be great if Java channels (e.g. `FileChannel`) were supported, too. Today I have to use a workaround: for an opened file channel, map the file into memory first, and then wrap the resulting `MappedByteBuffer` into `BufferedData`. Mapped buffers are not very trivial to maintain, sometimes it would be much simpler to wrap file channels directly.