AK: Delete bad pointer assignment operators and constructors.

We shouldn't allow constructing e.g an OwnPtr from a RefPtr, and similar
conversions. Instead just delete those functions so the compiler whines
loudly if you try to use them.

This patch also deletes constructing OwnPtr from a WeakPtr, even though
that *may* be a valid thing to do, it's sufficiently weird that we can
make the client jump through some hoops if he really wants it. :^)

AK/RefPtr.h

@@ -6,6 +6,9 @@
 
 namespace AK {
 
+template<typename T>
+class OwnPtr;
+
 template<typename T>
 class RefPtr {
 public:
@@ -86,6 +89,11 @@ public:
     }
     RefPtr(std::nullptr_t) {}
 
+    template<typename U>
+    RefPtr(const OwnPtr<U>&) = delete;
+    template<typename U>
+    RefPtr& operator=(const OwnPtr<U>&) = delete;
+
     RefPtr& operator=(RefPtr&& other)
     {
         if (this != &other) {