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/serialize.h

@@ -472,10 +472,10 @@ 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));
         }
     }
 
@@ -485,10 +485,10 @@ 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));
         }
     }