Introduce separate _clear functions for hash module

This gives the caller more control about whether the state should
be cleaned (= should be considered secret). Moreover, it gives the
caller the possibility to clean a hash struct without finalizing it.

src/modules/ecdh/main_impl.h

@@ -19,6 +19,7 @@ static int ecdh_hash_function_sha256(unsigned char *output, const unsigned char
     secp256k1_sha256_write(&sha, &version, 1);
     secp256k1_sha256_write(&sha, x32, 32);
     secp256k1_sha256_finalize(&sha, output);
+    secp256k1_sha256_clear(&sha);
 
     return 1;
 }

src/modules/ellswift/main_impl.h

@@ -510,6 +510,7 @@ static int ellswift_xdh_hash_function_prefix(unsigned char *output, const unsign
     secp256k1_sha256_write(&sha, ell_b64, 64);
     secp256k1_sha256_write(&sha, x32, 32);
     secp256k1_sha256_finalize(&sha, output);
+    secp256k1_sha256_clear(&sha);
 
     return 1;
 }
@@ -539,6 +540,7 @@ static int ellswift_xdh_hash_function_bip324(unsigned char* output, const unsign
     secp256k1_sha256_write(&sha, ell_b64, 64);
     secp256k1_sha256_write(&sha, x32, 32);
     secp256k1_sha256_finalize(&sha, output);
+    secp256k1_sha256_clear(&sha);
 
     return 1;
 }

src/modules/musig/session_impl.h

@@ -386,10 +386,10 @@ static void secp256k1_nonce_function_musig(secp256k1_scalar *k, const unsigned c
 
         /* Attempt to erase secret data */
         secp256k1_memclear(buf, sizeof(buf));
-        secp256k1_memclear(&sha_tmp, sizeof(sha_tmp));
+        secp256k1_sha256_clear(&sha_tmp);
     }
     secp256k1_memclear(rand, sizeof(rand));
-    secp256k1_memclear(&sha, sizeof(sha));
+    secp256k1_sha256_clear(&sha);
 }
 
 int secp256k1_musig_nonce_gen_internal(const secp256k1_context* ctx, secp256k1_musig_secnonce *secnonce, secp256k1_musig_pubnonce *pubnonce, const unsigned char *input_nonce, const unsigned char *seckey, const secp256k1_pubkey *pubkey, const unsigned char *msg32, const secp256k1_musig_keyagg_cache *keyagg_cache, const unsigned char *extra_input32) {

src/modules/schnorrsig/main_impl.h

@@ -93,6 +93,7 @@ static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *ms
     secp256k1_sha256_write(&sha, xonly_pk32, 32);
     secp256k1_sha256_write(&sha, msg, msglen);
     secp256k1_sha256_finalize(&sha, nonce32);
+    secp256k1_sha256_clear(&sha);
     return 1;
 }