Diagnose two silently violated template parameter requirements

Both were on the list of requirements that are not caught at compile time and
corrupt values rather than failing, and both are a plain size comparison:

- A BinaryType whose value_type is wider than one byte, which the readers and
  writers reinterpret as raw bytes anyway.
- A NumberUnsignedType too narrow to hold the absolute value of every
  NumberIntegerType value, which makes basic_json(INT64_MIN).dump() yield -0
  for std::int64_t with std::uint32_t.

Neither static_assert rejects a configuration that worked before: both only
fire where the result was already wrong. Also add the two comments the review
asked for, in write_bson_string() and calc_bson_array_size(), matching the
ones their counterparts already carry.

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

include/nlohmann/detail/output/binary_writer.hpp

@@ -1060,6 +1060,8 @@ class binary_writer
         oa->write_characters(
             reinterpret_cast<const CharType*>(value.data()),
             value.size());
+        // the terminating null byte is written explicitly rather than taken
+        // from the buffer, so that string_t::data() need not be null-terminated
         oa->write_character(to_char_type(0x00));
     }
 
@@ -1151,6 +1153,9 @@ class binary_writer
 
         const std::size_t embedded_document_size = std::accumulate(std::begin(value), std::end(value), static_cast<std::size_t>(0), [&array_index](std::size_t result, const typename BasicJsonType::array_t::value_type & el)
         {
+            // the index is built as a std::string, while calc_bson_element_size
+            // takes a string_t; convert explicitly, as the two are only
+            // implicitly convertible for some string types
             const auto key = std::to_string(array_index++);
             return result + calc_bson_element_size(string_t(key.data(), key.size()), el);
         });