message: Make use of TAG_PUBKEY constants in secp256k1_eckey_pubkey_parse
  Summary:
  This results in more self-documenting code.
  This is a backport of secp256k1's PR562
  Test Plan:
  make check
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -18,7 +18,7 @@ static int secp256k1_eckey_pubkey_parse(secp256k1_ge *elem, const unsigned char
  if (size == 33 && (pub[0] == SECP256K1_TAG_PUBKEY_EVEN || pub[0] == SECP256K1_TAG_PUBKEY_ODD)) {
  secp256k1_fe x;
  return secp256k1_fe_set_b32(&x, pub+1) && secp256k1_ge_set_xo_var(elem, &x, pub[0] == SECP256K1_TAG_PUBKEY_ODD);
  - } else if (size == 65 && (pub[0] == 0x04 || pub[0] == 0x06 || pub[0] == 0x07)) {
  + } else if (size == 65 && (pub[0] == SECP256K1_TAG_PUBKEY_UNCOMPRESSED || pub[0] == SECP256K1_TAG_PUBKEY_HYBRID_EVEN || pub[0] == SECP256K1_TAG_PUBKEY_HYBRID_ODD)) {
  secp256k1_fe x, y;
  if (!secp256k1_fe_set_b32(&x, pub+1) || !secp256k1_fe_set_b32(&y, pub+33)) {
  return 0;
  