custom allocators: define missing 'rebind' type

`gcc-13` added an assert to standard headers to make sure custom
allocators have intended implementation of rebind type instead
of inherited rebind. gcc change:
    https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=64c986b49558a7

Without the fix build fails on this week's `gcc-13` as:

    In file included from <<NIX>>-gcc-13.0.0/include/c++/13.0.0/ext/alloc_traits.h:34,
                     from <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/basic_string.h:39,
                     from <<NIX>>-gcc-13.0.0/include/c++/13.0.0/string:54,
                     from <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/locale_classes.h:40,
                     from <<NIX>>-gcc-13.0.0/include/c++/13.0.0/locale:41,
                     from tests/src/unit-regression2.cpp:19:
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h: In instantiation of 'struct std::__allocator_traits_base::__rebind<my_allocator<unsigned char>, unsigned char, void>':
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h:94:11:   required by substitution of 'template<class _Alloc, class _Up> using std::__alloc_rebind = typename std::__allocator_traits_base::__rebind<_Alloc, _Up>::type [with _Alloc = my_allocator<unsigned char>; _Up = unsigned char]'
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h:228:8:   required by substitution of 'template<class _Alloc> template<class _Tp> using std::allocator_traits< <template-parameter-1-1> >::rebind_alloc = std::__alloc_rebind<_Alloc, _Tp> [with _Tp = unsigned char; _Alloc = my_allocator<unsigned char>]'
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/ext/alloc_traits.h:126:65:   required from 'struct __gnu_cxx::__alloc_traits<my_allocator<unsigned char>, unsigned char>::rebind<unsigned char>'
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/stl_vector.h:88:21:   required from 'struct std::_Vector_base<unsigned char, my_allocator<unsigned char> >'
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/stl_vector.h:423:11:   required from 'class std::vector<unsigned char, my_allocator<unsigned char> >'
    tests/src/unit-regression2.cpp:807:63:   required from here
    <<NIX>>-gcc-13.0.0/include/c++/13.0.0/bits/alloc_traits.h:70:31: error: static assertion failed: allocator_traits<A>::rebind_alloc<A::value_type> must be A
       70 |                         _Tp>::value,
          |                               ^~~~~

The change adds trivial `rebind` definition with expected return type
and satisfies conversion requirements.

tests/src/unit-allocator.cpp

@@ -20,11 +20,20 @@ struct bad_allocator : std::allocator<T>
 {
     using std::allocator<T>::allocator;
 
+    bad_allocator() = default;
+    template<class U> bad_allocator(const bad_allocator<U>& /*unused*/) { }
+
     template<class... Args>
     void construct(T* /*unused*/, Args&& ... /*unused*/)
     {
         throw std::bad_alloc();
     }
+
+    template <class U>
+    struct rebind
+    {
+        using other = bad_allocator<U>;
+    };
 };
 } // namespace
 

tests/src/unit-regression2.cpp

@@ -187,6 +187,15 @@ class my_allocator : public std::allocator<T>
 {
   public:
     using std::allocator<T>::allocator;
+
+    my_allocator() = default;
+    template<class U> my_allocator(const my_allocator<U>& /*unused*/) { }
+
+    template <class U>
+    struct rebind
+    {
+        using other = my_allocator<U>;
+    };
 };
 
 /////////////////////////////////////////////////////////////////////