Remove unreachable code found by the coverage analysis

Remove code that no input can reach, and mark the lines that cannot be
covered with LCOV_EXCL_LINE:

- BJData writer: the dry_run parameter of the ND-array element writers was
  always true; replace them by range checks.
- BON8 writer: call throw_integer_out_of_range() directly instead of
  write_bon8_value() for values that always throw.
- binary_writer: integer_bits() is only called for integers of up to 64 bits.
- serializer, view serializer, lexer: replace guards that the callers make
  unreachable by assertions or by the simpler code (surrogate pairs under
  ensure_ascii, next_unget, '-' at the end of the buffer).
- json_view: drop unreachable branches in scalar(), set(), check_utf8_scan(),
  append_input_impl() and unescape().
- Mark exception-cleanup braces, 2 GiB limits, big-endian and 32-bit code, and
  fall-through case labels with LCOV_EXCL_LINE.

No behavior changes for any input that reaches these functions.

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

include/nlohmann/detail/string_utils.hpp

@@ -36,7 +36,7 @@ StringType to_string(std::size_t value)
     StringType result;
     int_to_string(result, value);
     return result;
-}
+} // LCOV_EXCL_LINE (exception cleanup only)
 
 /// @return a byte as two uppercase hexadecimal digits
 inline std::string hex_byte(const std::uint8_t byte)
@@ -46,7 +46,7 @@ inline std::string hex_byte(const std::uint8_t byte)
     result[0] = nibble_to_hex[byte / 16];
     result[1] = nibble_to_hex[byte % 16];
     return result;
-}
+} // LCOV_EXCL_LINE (exception cleanup only)
 
 /*!
 @brief write a UTF-16 code unit as four lowercase hexadecimal digits
@@ -390,7 +390,7 @@ inline StringType sanitize_utf8(const StringType& s, const error_handler_t error
     }
 
     return result;
-}
+} // LCOV_EXCL_LINE (exception cleanup only)
 
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END