LibJS: Stop making shapes unique

We previously had a concept of unique shapes, which meant that they
couldn't be shared between multiple objects.

Object shapes became unique in three situations:

- They were the shape of the global object.
- They had more than 100 properties added to them.
- They had one or more properties deleted from them.

Unfortunately, unique shapes presented an annoying problem for inline
caches, and we added a "unique shape serial number" for being able to
tell that a unique shape had been mutated.

This patch gets rid of the concept of unique shapes, simplifying all
the caching code, since inline caches can now simply perform a shape
check and then we're good.

To make this possible, we now have the concept of delete transitions,
which occur when a property is deleted from a shape.

Note that this patch by itself introduces a performance regression in
some situtations, since we now create a lot more shapes, and marking
their property keys can be very heavy. This will be addressed in a
subsequent patch.

Userland/Libraries/LibJS/Runtime/Object.h

@@ -49,7 +49,6 @@ struct CacheablePropertyMetadata {
     };
     Type type { Type::NotCacheable };
     Optional<u32> property_offset;
-    u64 unique_shape_serial_number { 0 };
 };
 
 class Object : public Cell {
@@ -215,8 +214,6 @@ public:
 
     static FlatPtr shape_offset() { return OFFSET_OF(Object, m_shape); }
 
-    void ensure_shape_is_unique();
-
     template<typename T>
     bool fast_is() const = delete;