crypto: Refactor (P)RNG functionality into new crypto_rand module.

* ADD new /src/common/crypto_rand.[ch] module.
 * ADD new /src/common/crypto_util.[ch] module (contains the memwipe()
   function, since all crypto_* modules need this).
 * FIXES part of #24658: https://bugs.torproject.org/24658

src/tools/tor-gencert.c

@@ -40,6 +40,8 @@ ENABLE_GCC_WARNING(redundant-decls)
 #include "torlog.h"
 #include "crypto.h"
 #include "crypto_digest.h"
+#include "crypto_rand.h"
+#include "crypto_util.h"
 #include "address.h"
 #include "util_format.h"