AK: Add global FlatPtr typedef. It's u32 or u64, based on sizeof(void*)

Use this instead of uintptr_t throughout the codebase. This makes it
possible to pass a FlatPtr to something that has u32 and u64 overloads.

AK/HashFunctions.h

@@ -51,7 +51,7 @@ inline unsigned u64_hash(u64 key)
     return pair_int_hash(first, last);
 }
 
-inline unsigned ptr_hash(uintptr_t ptr)
+inline unsigned ptr_hash(FlatPtr ptr)
 {
     if constexpr(sizeof(ptr) == 8)
         return u64_hash((u64)ptr);
@@ -61,5 +61,5 @@ inline unsigned ptr_hash(uintptr_t ptr)
 
 inline unsigned ptr_hash(const void* ptr)
 {
-    return ptr_hash((uintptr_t)(ptr));
+    return ptr_hash((FlatPtr)(ptr));
 }