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/bench/ellswift.cpp

@@ -17,12 +17,12 @@ static void EllSwiftCreate(benchmark::Bench& bench)
     uint256 entropy = GetRandHash();
 
     bench.batch(1).unit("pubkey").run([&] {
-        auto ret = key.EllSwiftCreate(AsBytes(Span{entropy}));
+        auto ret = key.EllSwiftCreate(MakeByteSpan(entropy));
         /* Use the first 32 bytes of the ellswift encoded public key as next private key. */
         key.Set(UCharCast(ret.data()), UCharCast(ret.data()) + 32, true);
         assert(key.IsValid());
         /* Use the last 32 bytes of the ellswift encoded public key as next entropy. */
-        std::copy(ret.begin() + 32, ret.begin() + 64, AsBytePtr(entropy.data()));
+        std::copy(ret.begin() + 32, ret.begin() + 64, MakeWritableByteSpan(entropy).begin());
     });
 
     ECC_Stop();