schnorrsig: add extra parameter struct for sign_custom

This simplifies the interface of sign_custom and allows adding more parameters
later in a backward compatible way.

src/modules/schnorrsig/tests_exhaustive_impl.h

@@ -141,6 +141,8 @@ static void test_exhaustive_schnorrsig_verify(const secp256k1_context *ctx, cons
 static void test_exhaustive_schnorrsig_sign(const secp256k1_context *ctx, unsigned char (*xonly_pubkey_bytes)[32], const secp256k1_keypair* keypairs, const int* parities) {
     int d, k;
     uint64_t iter = 0;
+    secp256k1_schnorrsig_extraparams extraparams = SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT;
+
     /* Loop over keys. */
     for (d = 1; d < EXHAUSTIVE_TEST_ORDER; ++d) {
         int actual_d = d;
@@ -153,6 +155,8 @@ static void test_exhaustive_schnorrsig_sign(const secp256k1_context *ctx, unsign
             unsigned char sig64[64];
             int actual_k = k;
             if (skip_section(&iter)) continue;
+            extraparams.noncefp = secp256k1_hardened_nonce_function_smallint;
+            extraparams.ndata = &k;
             if (parities[k - 1]) actual_k = EXHAUSTIVE_TEST_ORDER - k;
             /* Generate random messages until all challenges have been tried. */
             while (e_count_done < EXHAUSTIVE_TEST_ORDER) {
@@ -165,7 +169,7 @@ static void test_exhaustive_schnorrsig_sign(const secp256k1_context *ctx, unsign
                     unsigned char expected_s_bytes[32];
                     secp256k1_scalar_get_b32(expected_s_bytes, &expected_s);
                     /* Invoke the real function to construct a signature. */
-                    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig64, msg32, sizeof(msg32), &keypairs[d - 1], secp256k1_hardened_nonce_function_smallint, &k));
+                    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig64, msg32, sizeof(msg32), &keypairs[d - 1], &extraparams));
                     /* The first 32 bytes must match the xonly pubkey for the specified k. */
                     CHECK(secp256k1_memcmp_var(sig64, xonly_pubkey_bytes[k - 1], 32) == 0);
                     /* The last 32 bytes must match the expected s value. */