science/py-dlib: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes science/py-dlib to fail to compile with clang 19 and libc++
19, resulting in errors similar to:

  /usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned int>'
    820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
        |                                          ^
  /wrkdirs/usr/ports/science/py-dlib/work-py311/dlib-19.24.6/dlib/bigint/../unicode/unicode.h:718:19: note: in instantiation of template class 'std::basic_string<unsigned int>' requested here
    718 |     const ustring convert_to_utf32 (
        |                   ^
  /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
     23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
        |                             ^

This can be fixed by defining `unichar` as `char32_t` instead of
`uint32` in `unicode.h`, and adding a serializer for `char32_t` to
`serialize.h`.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

PR:		282524
Approved by:	arrowd (maintainer)
MFH:		2024Q4