Add reference implementation for ntor, plus compatibility test

Before I started coding ntor in C, I did another one in Python.
Turns out, they interoperate just fine.

src/or/onion_ntor.c

@@ -3,26 +3,12 @@
 
 #include "orconfig.h"
 
-#include "onion_ntor.h"
 #include "crypto.h"
+#define ONION_NTOR_PRIVATE
+#include "onion_ntor.h"
 #include "torlog.h"
 #include "util.h"
 
-/** Storage held by a client while waiting for an ntor reply from a server. */
-struct ntor_handshake_state_t {
-  /** Identity digest of the router we're talking to. */
-  uint8_t router_id[DIGEST_LEN];
-  /** Onion key of the router we're talking to. */
-  curve25519_public_key_t pubkey_B;
-
-  /**
-   * Short-lived keypair for use with this handshake.
-   * @{ */
-  curve25519_secret_key_t seckey_x;
-  curve25519_public_key_t pubkey_X;
-  /** @} */
-};
-
 /** Free storage held in an ntor handshake state. */
 void
 ntor_handshake_state_free(ntor_handshake_state_t *state)

src/or/onion_ntor.h

@@ -38,6 +38,25 @@ int onion_skin_ntor_client_handshake(
                              const uint8_t *handshake_reply,
                              uint8_t *key_out,
                              size_t key_out_len);
+
+#ifdef ONION_NTOR_PRIVATE
+
+/** Storage held by a client while waiting for an ntor reply from a server. */
+struct ntor_handshake_state_t {
+  /** Identity digest of the router we're talking to. */
+  uint8_t router_id[DIGEST_LEN];
+  /** Onion key of the router we're talking to. */
+  curve25519_public_key_t pubkey_B;
+
+  /**
+   * Short-lived keypair for use with this handshake.
+   * @{ */
+  curve25519_secret_key_t seckey_x;
+  curve25519_public_key_t pubkey_X;
+  /** @} */
+};
+#endif
+
 #endif
 
 #endif