LibJS: Implement basic conservative garbage collection

We now scan the stack and CPU registers for potential pointers into the
GC heap, and include any valid Cell pointers in the set of roots.

This works pretty well but we'll also need to solve marking of things
passed to native functions, since those are currently in Vector<Value>
and the Vector storage is on the heap (not scanned.)

Libraries/LibJS/Heap/HeapBlock.h

@@ -61,6 +61,14 @@ public:
         return reinterpret_cast<HeapBlock*>((FlatPtr)cell & ~(block_size - 1));
     }
 
+    Cell* cell_from_possible_pointer(FlatPtr pointer)
+    {
+        if (pointer < reinterpret_cast<FlatPtr>(m_storage))
+            return nullptr;
+        size_t cell_index = (pointer - reinterpret_cast<FlatPtr>(m_storage)) / m_cell_size;
+        return cell(cell_index);
+    }
+
 private:
     HeapBlock(Heap&, size_t cell_size);