Add TaprootBuilder class

This class functions as a utility for building taproot outputs, from
internal key and script leaves.

src/pubkey.cpp

@@ -180,6 +180,12 @@ XOnlyPubKey::XOnlyPubKey(Span<const unsigned char> bytes)
     std::copy(bytes.begin(), bytes.end(), m_keydata.begin());
 }
 
+bool XOnlyPubKey::IsFullyValid() const
+{
+    secp256k1_xonly_pubkey pubkey;
+    return secp256k1_xonly_pubkey_parse(secp256k1_context_verify, &pubkey, m_keydata.data());
+}
+
 bool XOnlyPubKey::VerifySchnorr(const uint256& msg, Span<const unsigned char> sigbytes) const
 {
     assert(sigbytes.size() == 64);