refactor: Take Span in SetSeed

This makes calling code less verbose and less fragile. Also, by adding
the CKey::data() member function, it is now possible to call HexStr()
with a CKey object.

src/wallet/scriptpubkeyman.cpp

@@ -400,7 +400,7 @@ void LegacyScriptPubKeyMan::UpgradeKeyMetadata()
             CKey key;
             GetKey(meta.hd_seed_id, key);
             CExtKey masterKey;
-            masterKey.SetSeed(key.begin(), key.size());
+            masterKey.SetSeed(key);
             // Add to map
             CKeyID master_id = masterKey.key.GetPubKey().GetID();
             std::copy(master_id.begin(), master_id.begin() + 4, meta.key_origin.fingerprint);
@@ -1085,7 +1085,7 @@ void LegacyScriptPubKeyMan::DeriveNewChildKey(WalletBatch &batch, CKeyMetadata&
     if (!GetKey(hd_chain.seed_id, seed))
         throw std::runtime_error(std::string(__func__) + ": seed not found");
 
-    masterKey.SetSeed(seed.begin(), seed.size());
+    masterKey.SetSeed(seed);
 
     // derive m/0'
     // use hardened derivation (child keys >= 0x80000000 are hardened after bip32)