Add serialization methods to XOnlyPubKey

It is useful to have serialzation methods for XOnlyPubKey. These will
serialize the internal uint256, so it is not prefixed with the length as
CPubKey does.

Author: achow101

src/pubkey.h

@@ -286,6 +286,9 @@ class XOnlyPubKey
     bool operator==(const XOnlyPubKey& other) const { return m_keydata == other.m_keydata; }
     bool operator!=(const XOnlyPubKey& other) const { return m_keydata != other.m_keydata; }
     bool operator<(const XOnlyPubKey& other) const { return m_keydata < other.m_keydata; }
+
+    //! Implement serialization without length prefixes since it is a fixed length
+    SERIALIZE_METHODS(XOnlyPubKey, obj) { READWRITE(obj.m_keydata); }
 };
 
 struct CExtPubKey {