Address review findings on the binary writer output sinks

- binary_reserve_hint(): the 4-bytes-per-element estimate over-reserved by up
  to 4x for arrays of small scalars (CBOR encodes 0..23 in one byte), and the
  returned vector kept that capacity. Make the hint a strict lower bound on the
  encoded size instead, which also removes the 1 MiB clamp whose branch no test
  could reach (the largest container in the suite has 65793 elements).

- Guard the -Wduplicated-branches pragma with __GNUC__ >= 7. The warning does
  not exist before GCC 7, so naming it made GCC 4.8/4.9/5/6 - which the CI
  matrix still builds - warn under -Wpragmas on every including translation
  unit, breaking downstream -Werror builds.

- Constrain the adapter constructor of binary_writer with the enable_if its
  documentation already claimed, so a writer over some other sink type is no
  longer advertised as constructible from an output adapter.

- Let output_vector_adapter wrap output_vector_sink rather than duplicating the
  append logic, so the type-erased and templated paths share one implementation.

- Collapse the three copies of the memcpy/byte_swap/memcpy dance into a single
  byte_swap_buffer() helper, and add the MSVC _byteswap_* intrinsics so MSVC no
  longer falls back to the scalar shuffle this change exists to eliminate.

- Add a vector_writer() helper for the five vector-returning to_* overloads
  instead of spelling out the writer type at each call site, and drop a dead
  default member initializer on output_adapter_sink.

- New tests: the vector sink and the adapter sink must produce identical bytes
  for every format (the two to_* overloads no longer delegate to each other and
  could otherwise drift), and binary_reserve_hint() must never exceed the size
  actually written.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -187,6 +187,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     template<typename InputType>
     using binary_reader = ::nlohmann::detail::binary_reader<basic_json, InputType>;
     template<typename CharType> using binary_writer = ::nlohmann::detail::binary_writer<basic_json, CharType>;
+    // binary_writer over a concrete (non-virtual) sink appending into a std::vector,
+    // used by the vector-returning to_* overloads
+    template<typename CharType> using vector_binary_writer =
+        ::nlohmann::detail::binary_writer<basic_json, CharType, ::nlohmann::detail::output_vector_sink<CharType>>;
+    template<typename CharType> static vector_binary_writer<CharType> vector_writer(std::vector<CharType>& v)
+    {
+        return vector_binary_writer<CharType>(::nlohmann::detail::output_vector_sink<CharType>(v));
+    }
 
   JSON_PRIVATE_UNLESS_TESTED:
     using serializer = ::nlohmann::detail::serializer<basic_json>;
@@ -4338,8 +4346,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         std::vector<std::uint8_t> result;
         result.reserve(detail::binary_reserve_hint(j));
-        detail::binary_writer<basic_json, std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
-            detail::output_vector_sink<std::uint8_t>(result)).write_cbor(j);
+        vector_writer(result).write_cbor(j);
         return result;
     }
 
@@ -4363,8 +4370,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         std::vector<std::uint8_t> result;
         result.reserve(detail::binary_reserve_hint(j));
-        detail::binary_writer<basic_json, std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
-            detail::output_vector_sink<std::uint8_t>(result)).write_msgpack(j);
+        vector_writer(result).write_msgpack(j);
         return result;
     }
 
@@ -4390,8 +4396,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         std::vector<std::uint8_t> result;
         result.reserve(detail::binary_reserve_hint(j));
-        detail::binary_writer<basic_json, std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
-            detail::output_vector_sink<std::uint8_t>(result)).write_ubjson(j, use_size, use_type);
+        vector_writer(result).write_ubjson(j, use_size, use_type);
         return result;
     }
 
@@ -4420,8 +4425,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         std::vector<std::uint8_t> result;
         result.reserve(detail::binary_reserve_hint(j));
-        detail::binary_writer<basic_json, std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
-            detail::output_vector_sink<std::uint8_t>(result)).write_ubjson(j, use_size, use_type, true, true, version);
+        vector_writer(result).write_ubjson(j, use_size, use_type, true, true, version);
         return result;
     }
 
@@ -4449,8 +4453,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         std::vector<std::uint8_t> result;
         result.reserve(detail::binary_reserve_hint(j));
-        detail::binary_writer<basic_json, std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
-            detail::output_vector_sink<std::uint8_t>(result)).write_bson(j);
+        vector_writer(result).write_bson(j);
         return result;
     }