AK: Add ConstPeekType to Traits

Also, the PeekType of smart pointers is now T* instead of const T*.

Note: This commit doesn't compile, it breaks HashMap::get() for some
types. Fixed in the next commit.

AK/Traits.h

@@ -14,6 +14,7 @@ namespace AK {
 template<typename T>
 struct GenericTraits {
     using PeekType = T;
+    using ConstPeekType = T;
     static constexpr bool is_trivial() { return false; }
     static constexpr bool equals(const T& a, const T& b) { return a == b; }
 };