shared_ptr::unique not available anymore in C++20

[lgeretti]

Apparently the method has been removed, but this shows up only using MSVC.
We should address this, since it is used in Handle. I currently work around it by just not checking for uniqueness and making a unique one anyway.

[pietercollins]

In Handle<const I> we can safely used shared_ptr without copying.

For non-const Handle<I>, if we can't rely on shared_ptr::unique then we either need to copy on every non-constant access (horrible) or make a deep copy when the handle is copied. This seems like the only sensible solution. We then might as well use unique_ptr. Of course, we can pass the object by reference to avoid copying, and we can use move-constructors where possible.

I suggest we make a template specialisation of Handle<const I> using shared_ptr and use unique_ptr in Handle<I>.