clean up stdio_impl.h

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.

now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.

src/stdio/open_wmemstream.c

@@ -1,4 +1,8 @@
 #include "stdio_impl.h"
+#include <wchar.h>
+#include <errno.h>
+#include <limits.h>
+#include <string.h>
 
 struct cookie {
 	wchar_t **bufp;