LibC: Implement almost POSIX compliant *scanf functions

Only wchar strings and floating point values are not parsed

libc/include/scanf_impl.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include <stdarg.h>
+
+int scanf_impl(const char* format, va_list arguments, int (*getc_fun)(void*), void* data);