Kernel: Make Region single-owner instead of ref-counted

This simplifies the ownership model and makes Region easier to reason
about. Userspace Regions are now primarily kept by Process::m_regions.

Kernel Regions are kept in various OwnPtr<Regions>'s.

Regions now only ever get unmapped when they are destroyed.

AK/ELF/ELFLoader.h

@@ -54,7 +54,7 @@ private:
         const char* name;
     };
 #ifdef KERNEL
-    mutable RefPtr<Region> m_sorted_symbols_region;
+    mutable OwnPtr<Region> m_sorted_symbols_region;
 #else
     mutable Vector<SortedSymbol> m_sorted_symbols;
 #endif