Add allocator::rebind for c++20

[SilverTuxedo]

The rebind member has been removed from std::allocator in c++20 following
its deprecation in c++17. However, since the jxy allocator cannot be constructed
like the default allocator due to its custom template parameters, it must define
this member.

Fixes #2

[jxy-s]

rebind was deprecated in cpp17 and removed in cpp20 I'm a little weary to accept this change because of this, allow me some time to investigate

[jxy-s]

Okay, so I did a little digging and while rebind is deprecated on std::allocator it may be still required for this custom allocator:

https://en.cppreference.com/w/cpp/named_req/Allocator#cite_note-2

↑ rebind is only optional (provided by std::allocator_traits) if this allocator is a template of the form SomeAllocator<T, Args>, where Args is zero or more additional template type parameters.

That said, I'm a little perplexed why std::_Replace_first_parameter<_Other,_Ty> is tripping over this:

3>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xmemory(469,66): error C2027: use of undefined type 'std::_Replace_first_parameter<_Other,_Ty>'
3>        with
3>        [
3>            _Other=int,
3>            _Ty=jxy::details::allocator<int,PagedPool,809976148>
3>        ]

Let's at least comment/document the reason for this change so when someone goes to look at this later they understand why jxy::allocator has rebind functionality that doesn't mirror std::allocator deprecating it.