Take a `shared_ptr<widget>` parameter to express shared ownership

Reason

This makes the function's ownership sharing explicit.

Example, good

class WidgetUser
{
public:
    // WidgetUser will share ownership of the widget
    explicit WidgetUser(std::shared_ptr<widget> w) noexcept:
        m_widget{std::move(w)} {}
    // ...
private:
    std::shared_ptr<widget> m_widget;
};

Enforcement

• (Simple) Warn if a function takes a Shared_pointer<T> parameter by lvalue reference and does not either assign to it or call reset() on it on at least one code path. Suggest taking a T* or T& instead.
• (Simple) ((Foundation)) Warn if a function takes a Shared_pointer<T> by value or by reference to const and does not copy or move it to another Shared_pointer on at least one code path. Suggest taking a T* or T& instead.
• (Simple) ((Foundation)) Warn if a function takes a Shared_pointer<T> by rvalue reference. Suggesting taking it by value instead.