uv_type: make ~uv_type() destructor protected

Make the destructor for `uv_type` `protected`, in order to the fix the
`-Wnon-virtual-dtor` warnings:

```
‘struct uvw::uv_type<uv_async_s>’ has virtual functions and accessible non-virtual destructor [-Werror=non-virtual-dtor]
```

As an example, the code below currently works (casting a pointer to a
subclass of `uvw::uv_type` to `uvw::uv_type`, but it causes a memory
leak, since only the `uvw::uv_type` destructor is called.

Doing this is now forbidden.

```c++
uvw::resource<...>* will_leak;
auto abc = std::unique_ptr<uvw::uv_type>(will_leak);
```

BREAKING CHANGE: This change may break the uvw ABI.
                 It also breaks the API.

src/uvw/uv_type.hpp

@@ -79,6 +79,9 @@ struct uv_type {
         return &resource;
     }
 
+protected:
+    ~uv_type() = default;
+
 private:
     std::shared_ptr<loop> owner;
     U resource;