TODO: integrate ByteBuffer better with FileHandle and Socket

[moritz]

It would be nice if the ByteBuffer PMC was better integrated with other subsystems. In particular

• the FileHandle has a read_bytes method, but no corresponding write_bytes (or .print() or whatever). Direct binary write operations on FileHandle would be most welcome.
• the Socket PMC would benefit from a read_bytes method too (or return a ByteBuffer from read(), but that's not backwards compatible)
• likewise writing a ByteBuffer to a Socket