Uncoupling from Buffer

[back2dos]

Currently, much of the library depends on Buffer, in particular Source.read and Sink.write could just accept (bytes, offset, max) instead of the buffer. That might make it easier to integrate with other libraries. Now that things are starting to work correctly, I don't want to break everything again, but when Haxe 4 comes it, it might be a good chance to reevaluate this possibility.

[back2dos]

Part of #11 now.