Merge bitcoin/bitcoin#27978: refactor: Drop unsafe AsBytePtr function

7c853619ee refactor: Drop unsafe AsBytePtr function (Ryan Ofsky)

Pull request description:

  Replace calls to `AsBytePtr` with calls to `AsBytes` or `reinterpret_cast`. `AsBytePtr` is just a wrapper around `reinterpret_cast`. It accepts any type of pointer as an argument and uses `reinterpret_cast` to cast the argument to a `std::byte` pointer.

  Despite taking any type of pointer as an argument, it is not useful to call `AsBytePtr` on most types of pointers, because byte representations of most types will be platform specific or undefined. Also, because it is named similarly to the `AsBytes` function, `AsBytePtr` looks safer than it actually is. Both `AsBytes` and `AsBytePtr` call reinterpret_cast internally and may be unsafe to use with certain types, but AsBytes at least has some type checking and can only be called on `Span` objects, while `AsBytePtr` can be called on any pointer argument.

  The change was motivated by discussion on #27973 and #27927 and is compatible with those PRs

ACKs for top commit:
  jonatack:
    re-ACK 7c853619ee
  sipa:
    utACK 7c853619ee
  achow101:
    ACK 7c853619ee

Tree-SHA512: 200d858b1d4d579f081a7f9a14d488a99713b4918b4564ac3dd5c18578d927dbd6426e62e02f49f04a3fa73ca02ff7109c495cb0b92bec43c27d9b74e2f95757

src/serialize.h

@@ -473,10 +473,10 @@ struct CustomUintFormatter
         if (v < 0 || v > MAX) throw std::ios_base::failure("CustomUintFormatter value out of range");
         if (BigEndian) {
             uint64_t raw = htobe64(v);
-            s.write({AsBytePtr(&raw) + 8 - Bytes, Bytes});
+            s.write(AsBytes(Span{&raw, 1}).last(Bytes));
         } else {
             uint64_t raw = htole64(v);
-            s.write({AsBytePtr(&raw), Bytes});
+            s.write(AsBytes(Span{&raw, 1}).first(Bytes));
         }
     }
 
@@ -486,10 +486,10 @@ struct CustomUintFormatter
         static_assert(std::numeric_limits<U>::max() >= MAX && std::numeric_limits<U>::min() <= 0, "Assigned type too small");
         uint64_t raw = 0;
         if (BigEndian) {
-            s.read({AsBytePtr(&raw) + 8 - Bytes, Bytes});
+            s.read(AsWritableBytes(Span{&raw, 1}).last(Bytes));
             v = static_cast<I>(be64toh(raw));
         } else {
-            s.read({AsBytePtr(&raw), Bytes});
+            s.read(AsWritableBytes(Span{&raw, 1}).first(Bytes));
             v = static_cast<I>(le64toh(raw));
         }
     }