Merge bitcoin/bitcoin#27927: util: Allow std::byte and char Span serialization

fa38d86235 Use only Span{} constructor for byte-like types where possible (MarcoFalke)
fa257bc831 util: Allow std::byte and char Span serialization (MarcoFalke)

Pull request description:

  Seems odd to require developers to cast all byte-like spans passed to serialization to `unsigned char`-spans. Fix that by passing and accepting byte-like spans as-is. Finally, add tests and update the code to use just `Span` where possible.

ACKs for top commit:
  sipa:
    utACK fa38d86235
  achow101:
    ACK fa38d86235
  ryanofsky:
    Code review ACK fa38d86235. This looks great. The second commit really removes a lot of boilerplate and shows why the first commit is useful.

Tree-SHA512: 788592d9ff515c3ebe73d48f9ecbb8d239f5b985af86f09974e508cafb0ca6d73a959350295246b4dfb496149bc56330a0b5d659fc434ba6723dbaba0b7a49e5

src/wallet/walletdb.cpp

@@ -1392,7 +1392,7 @@ bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
         }
 
         // Make a copy of key to avoid data being deleted by the following read of the type
-        Span<const unsigned char> key_data = MakeUCharSpan(key);
+        Span key_data{key};
 
         std::string type;
         key >> type;