Add new implementation unit System.Native_IO, and unify 2-version Ada.Streams.Stream_IO.Inside.Standard_Files into one
Delete unnecessary pragma Elaborate_Body
Delete unnecessary constant
Move platform-depended file name operations from the implementation unit Ada.Stream_IO.Inside to System.Native_IO
Move platform-depended file management operations from the implementation unit Ada.Stream_IO.Inside to System.Native_IO
Move remaining platform-depended operations from the implementation unit Ada.Stream_IO.Inside to System.Native_IO, and unify 2-version Ada.Streams.Stream_IO.Inside into one