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/test/bip32_tests.cpp

@@ -124,7 +124,7 @@ void RunTest(const TestVector &test) {
     std::vector<unsigned char> seed = ParseHex(test.strHexMaster);
     CExtKey key;
     CExtPubKey pubkey;
-    key.SetSeed(seed.data(), seed.size());
+    key.SetSeed(seed);
     pubkey = key.Neuter();
     for (const TestDerivation &derive : test.vDerive) {
         unsigned char data[74];