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/GlobalObject.cpp

@@ -92,7 +92,6 @@ JS_DEFINE_ALLOCATOR(GlobalObject);
 GlobalObject::GlobalObject(Realm& realm)
     : Object(GlobalObjectTag::Tag, realm)
 {
-    ensure_shape_is_unique();
     Object::set_prototype(realm.intrinsics().object_prototype());
 }