Implement invokable methods

Implement invokable methods, invokable constructors, and slots. Also provide method tags.

Ideally also detect accessiblity automatically, eventhough I have no idea yet how this could be achieve. Maybe a creative combination of macros and concepts might do the job. Something similar to this pseudo code:

``` C++
#define IS_PROTECTED(Class, Member) (requires { class Test : public Class { auto test() { return &Class::Member; } }; })
#define IS_PRIVATE(Class, Member) (std::is_member_function_pointer_v<&Class::Member> && !IS_PROTECTED(Class, Member))
```