Fix warnings

This change fixes Cosmopolitan so it has fewer opinions about compiler
warnings. The whole repository had to be cleaned up to be buildable in
-Werror -Wall mode. This lets us benefit from things like strict const
checking. Some actual bugs might have been caught too.

third_party/chibicc/dox1.c

@@ -42,7 +42,6 @@ struct DoxWriter {
 };
 
 static void SerializeData(struct Buffer *buf, const void *p, unsigned long n) {
-  struct Slice *s;
   buf->p = realloc(buf->p, buf->n + n);
   memcpy(buf->p + buf->n, p, n);
   buf->n += n;