Merge bitcoin/bitcoin#23438: refactor: Use spans of std::byte in serialize

fa5d2e678c Remove unused char serialize (MarcoFalke)
fa24493d63 Use spans of std::byte in serialize (MarcoFalke)
fa65bbf217 span: Add BytePtr helper (MarcoFalke)

Pull request description:

  This changes the serialize code (`.read()` and `.write()` functions) to take a `Span` instead of a pointer and size. This is a breaking change for the serialize interface, so at no additional cost we can also switch to `std::byte` (instead of using `char`).

  The benefits of using `Span`:
  * Less verbose and less fragile code when passing an already existing `Span`(-like) object to or from serialization

  The benefits of using `std::byte`:
  * `std::byte` can't accidentally be mistaken for an integer

  The goal here is to only change serialize to use spans of `std::byte`. If needed, `AsBytes`,  `MakeUCharSpan`, ... can be used (temporarily) to pass spans of the right type.

  Other changes that are included here:

  * [#22167](https://github.com/bitcoin/bitcoin/pull/22167) (refactor: Remove char serialize by MarcoFalke)
  * [#21906](https://github.com/bitcoin/bitcoin/pull/21906) (Preserve const in cast on CTransactionSignatureSerializer by promag)

ACKs for top commit:
  laanwj:
    Concept and code review ACK fa5d2e678c
  sipa:
    re-utACK fa5d2e678c

Tree-SHA512: 08ee9eced5fb777cedae593b11e33660bed9a3e1711a7451a87b835089a96c99ce0632918bb4666a4e859c4d020f88fb50f2dd734216b0c3d1a9a704967ece6f

src/script/interpreter.cpp

@@ -1303,12 +1303,12 @@ public:
         it = itBegin;
         while (scriptCode.GetOp(it, opcode)) {
             if (opcode == OP_CODESEPARATOR) {
-                s.write((char*)&itBegin[0], it-itBegin-1);
+                s.write(AsBytes(Span{&itBegin[0], size_t(it - itBegin - 1)}));
                 itBegin = it;
             }
         }
         if (itBegin != scriptCode.end())
-            s.write((char*)&itBegin[0], it-itBegin);
+            s.write(AsBytes(Span{&itBegin[0], size_t(it - itBegin)}));
     }
 
     /** Serialize an input of txTo */