Also, remove the Windows OsStack implementation. It will not ever
be useful in libfringe, as even if we add support for Windows
using fibers, the OS allocates stacks for fibers and we needn't
do it manually.
The new design concerns itself with one thing and exactly one thing:
passing values back and forth with an extern "C" function.
This allows to simplify fringe::arch into a single primitive, swap.
Close#21