Merge bitcoin/bitcoin#31540: refactor: std::span compat fixes

fa494a1d53 refactor: Specify const in std::span constructor, where needed (MarcoFalke)
faaf4800aa Allow std::span in stream serialization (MarcoFalke)
faa5391f77 refactor: test: Return std::span from StringBytes (MarcoFalke)
fa86223475 refactor: Avoid passing span iterators when data pointers are expected (MarcoFalke)
faae6fa5f6 refactor: Simplify SpanPopBack (MarcoFalke)
facc4f120b refactor: Replace fwd-decl with proper include (MarcoFalke)
fac3a782ea refactor: Avoid needless, unsafe c-style cast (MarcoFalke)

Pull request description:

  The `std::span` type is already used in some parts of the codebase, and in most contexts can implicitly convert to and from `Span`. However, the two types are not identical in behavior and trying to use one over the other can result in compile failures in some contexts.

  Fix all those issues by allowing either `Span` or `std::span` in any part of the codebase.

  All of the changes are also required for the scripted-diff to replace `Span` with `std::span` in https://github.com/bitcoin/bitcoin/pull/31519

ACKs for top commit:
  sipa:
    utACK fa494a1d53
  fjahr:
    Code review ACK fa494a1d53
  achow101:
    ACK fa494a1d53
  theuni:
    utACK fa494a1d53.
  adamandrews1:
    utACK fa494a1d53

Tree-SHA512: 9440941823e884ff5d7ac161f58b9a0704d8e803b4c91c400bdb5f58f898e4637d63ae627cfc7330e98a721fc38285a04641175aa18d991bd35f8b69ed1d74c4

src/wallet/test/db_tests.cpp

@@ -34,9 +34,9 @@ inline std::ostream& operator<<(std::ostream& os, const std::pair<const Serializ
 
 namespace wallet {
 
-static Span<const std::byte> StringBytes(std::string_view str)
+inline std::span<const std::byte> StringBytes(std::string_view str)
 {
-    return AsBytes<const char>({str.data(), str.size()});
+    return std::as_bytes(std::span{str});
 }
 
 static SerializeData StringData(std::string_view str)