Merge bitcoin/bitcoin#23451: span: Add std::byte helpers

faa3ec2304 span: Add std::byte helpers (MarcoFalke)
fa18038f51 refactor: Use ignore helper when unserializing an invalid pubkey (MarcoFalke)
fabe18d0b3 Use value_type in CDataStream where possible (MarcoFalke)

Pull request description:

  This adds (currently unused) span std::byte helpers, so that they can be used in new code.

  The refactors are also required for https://github.com/bitcoin/bitcoin/pull/23438, but they are split up because the other pull doesn't compile with msvc right now.

  The third commit is not needed for the other pull, but still nice.

ACKs for top commit:
  klementtan:
    reACK  faa3ec2. Verified that all the new `std::byte` helper functions are tested.
  laanwj:
    Code review ACK faa3ec2304

Tree-SHA512: b1f6af39f03ea4dfebf20d4a8538fa993a6104e7fc92ddf0c4606a7efc3ca9a8c1a4741d98a1418569c11bb9ce9258bf0c0c06d93d85ed7e208902a2db04e407

src/util/strencodings.cpp

@@ -139,11 +139,6 @@ std::string EncodeBase64(Span<const unsigned char> input)
     return str;
 }
 
-std::string EncodeBase64(const std::string& str)
-{
-    return EncodeBase64(MakeUCharSpan(str));
-}
-
 std::vector<unsigned char> DecodeBase64(const char* p, bool* pf_invalid)
 {
     static const int decode64_table[256] =