* Create `rz_windows.h`
* Move `RzThread` structure definitions to internal header
* Create and use `rz_th_get_user()` API to avoid exposing `RzThread` layout in external header
* Make `rz_th_self()` an IPI
* Use `_wmkdir()` in `rz_sys_mkdir()` on Windows
* Remove unused `winkd_break_read` function
* Remove `RzIORap` definition from `rz_io.h`
* Remove include of `Windows.h` from `rz_types.h`
* Use `rz_windows.h` when necessary
* Remove `RzCoreRtrHost` definition from `rz_core.h`
* Remove `rz_socket.h` include from `rz_core.h`
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