Header and source file structure

As part of the typed zip experiment I've started to play around with the structure of headers and source files, mainly for two reasons.

- Instead of passing `Dynamic` handles through the `_hx_` c functions the underlying classes are now exposed. But to avoid implementation details being leaked an abstract class of pure virtual functions is exposed (see `include/hx/zip/` folder).
- I moved the zlib implementation of the zip api into `src/hx/zip/zlib` instead of where it use to be so it matches the header folder more closely.

There's another thing I want to do which is to more formally expose those implementations, this would be especially useful for more fundamental types such as files and sockerts where you may want to get at the underlying OS handle to do stuff not exposed via the haxe api.
So would it be useful to haxe something like `include/hx/sys/File.hpp` and then `include/hx/sys/win32/Win32File.hpp` and `include/hx/sys/posix/PosixFile.hpp` style setup for this?