Replace PtrContainer with std::optional<std::reference_wrapper<T>>

[SiebrenW]

Hi. I noticed the FIXME in the ModuleAdapter.hpp asking whether the standard library had such a container. It does, but it's a combination of two.
I think this snippet can answer a lot more than me trying to explain it:
https://godbolt.org/z/zd1jjWb4h

The interface will be slightly different.