LibJS: Make it possible to go from a Cell* to its Heap&

This patch makes all HeapBlock allocations aligned to their block size,
enabling us to find the HeapBlock* for a given Cell* by simply masking
bits off of the cell address.

Use this to make a simple Heap& getter for Cell, which lets us avoid
plumbing the Heap& everywhere.

Libraries/LibJS/Cell.h

@@ -51,6 +51,8 @@ public:
 
     virtual void visit_children(Visitor&) {}
 
+    Heap& heap();
+
 private:
     bool m_mark { false };
     bool m_live { true };