refactor: Drop unsafe AsBytePtr function

Replace calls to AsBytePtr with direct 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 implmentation-specific. 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.

Co-authored-by: Pieter Wuille <pieter@wuille.net>

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();