Avoid a sha256/ripemd160 call for each pubkey we derive

We only take the pubkey and ignore all other fields, so we might as well
save the cycles used computing the hash for something else.

Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>

common/key_derive.c

@@ -248,13 +248,13 @@ bool derive_revocation_privkey(const struct secret *base_secret,
 bool bip32_pubkey(const struct ext_key *bip32_base,
 		  struct pubkey *pubkey, u32 index)
 {
+	const uint32_t flags = BIP32_FLAG_KEY_PUBLIC | BIP32_FLAG_SKIP_HASH;
 	struct ext_key ext;
 
 	if (index >= BIP32_INITIAL_HARDENED_CHILD)
 		return false;
 
-	if (bip32_key_from_parent(bip32_base, index,
-				  BIP32_FLAG_KEY_PUBLIC, &ext) != WALLY_OK)
+	if (bip32_key_from_parent(bip32_base, index, flags, &ext) != WALLY_OK)
 		return false;
 
 	if (!secp256k1_ec_pubkey_parse(secp256k1_ctx, &pubkey->pubkey,