Add missing XOnlyPubKey::data() to get mutable data

This is needed for consistency, and also to allow std::span construction
from XOnlyPubKey.

Author: MarcoFalke

src/pubkey.h

@@ -285,10 +285,11 @@ class XOnlyPubKey
     CPubKey GetEvenCorrespondingCPubKey() const;
 
     const unsigned char& operator[](int pos) const { return *(m_keydata.begin() + pos); }
-    const unsigned char* data() const { return m_keydata.begin(); }
     static constexpr size_t size() { return decltype(m_keydata)::size(); }
+    const unsigned char* data() const { return m_keydata.begin(); }
     const unsigned char* begin() const { return m_keydata.begin(); }
     const unsigned char* end() const { return m_keydata.end(); }
+    unsigned char* data() { return m_keydata.begin(); }
     unsigned char* begin() { return m_keydata.begin(); }
     unsigned char* end() { return m_keydata.end(); }
     bool operator==(const XOnlyPubKey& other) const { return m_keydata == other.m_keydata; }