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/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;
 }