Make it compilable with C++20

[kenichiice]

The following code will trigger a compile error when using C++20.

#include "nanort.h"

int main()
{
  nanort::StackVector<double, 100> v;
}

This is because 'nanort.h' uses the following, which have been removed in C++20 (deprecated since C++17).

• std::allocator<T>::pointer
• std::allocator<T>::allocate(size_type n, const void* hint)

This PR make changes to avoid using there.

This will allow the code shown at the beginning to compile using C++20.

[syoyo]

Awesome! Thanks!