Refactor VM root modules

This `st_table` is used to both mark and pin classes
defined from the C API. But `vm->mark_object_ary` already
does both much more efficiently.

Currently a Ruby process starts with 252 rooted classes,
which uses `7224B` in an `st_table` or `2016B` in an `RArray`.

So a baseline of 5kB saved, but since `mark_object_ary` is
preallocated with `1024` slots but only use `405` of them,
it's a net `7kB` save.

`vm->mark_object_ary` is also being refactored.

Prior to this changes, `mark_object_ary` was a regular `RArray`, but
since this allows for references to be moved, it was marked a second
time from `rb_vm_mark()` to pin these objects.

This has the detrimental effect of marking these references on every
minors even though it's a mostly append only list.

But using a custom TypedData we can save from having to mark
all the references on minor GC runs.

Addtionally, immediate values are now ignored and not appended
to `vm->mark_object_ary` as it's just wasted space.

class.c

@@ -32,6 +32,9 @@
 
 /* Flags of T_CLASS
  *
+ * 0:    RCLASS_IS_ROOT
+ *           The class has been added to the VM roots. Will always be marked and pinned.
+ *           This is done for classes defined from C to allow storing them in global variables.
  * 1:    RUBY_FL_SINGLETON
  *           This class is a singleton class.
  * 2:    RCLASS_SUPERCLASSES_INCLUDE_SELF
@@ -56,6 +59,9 @@
 
 /* Flags of T_MODULE
  *
+ * 0:    RCLASS_IS_ROOT
+ *           The class has been added to the VM roots. Will always be marked and pinned.
+ *           This is done for classes defined from C to allow storing them in global variables.
  * 1:    RMODULE_ALLOCATED_BUT_NOT_INITIALIZED
  *           Module has not been initialized.
  * 2:    RCLASS_SUPERCLASSES_INCLUDE_SELF
@@ -812,7 +818,7 @@ boot_defclass(const char *name, VALUE super)
     ID id = rb_intern(name);
 
     rb_const_set((rb_cObject ? rb_cObject : obj), id, obj);
-    rb_vm_add_root_module(obj);
+    rb_vm_register_global_object(obj);
     return obj;
 }
 
@@ -894,7 +900,7 @@ Init_class_hierarchy(void)
 {
     rb_cBasicObject = boot_defclass("BasicObject", 0);
     rb_cObject = boot_defclass("Object", rb_cBasicObject);
-    rb_gc_register_mark_object(rb_cObject);
+    rb_vm_register_global_object(rb_cObject);
 
     /* resolve class name ASAP for order-independence */
     rb_set_class_path_string(rb_cObject, rb_cObject, rb_fstring_lit("Object"));
@@ -988,14 +994,14 @@ rb_define_class(const char *name, VALUE super)
         }
 
         /* Class may have been defined in Ruby and not pin-rooted */
-        rb_vm_add_root_module(klass);
+        rb_vm_register_global_object(klass);
         return klass;
     }
     if (!super) {
         rb_raise(rb_eArgError, "no super class for '%s'", name);
     }
     klass = rb_define_class_id(id, super);
-    rb_vm_add_root_module(klass);
+    rb_vm_register_global_object(klass);
     rb_const_set(rb_cObject, id, klass);
     rb_class_inherited(super, klass);
 
@@ -1045,7 +1051,7 @@ VALUE
 rb_define_class_id_under(VALUE outer, ID id, VALUE super)
 {
     VALUE klass = rb_define_class_id_under_no_pin(outer, id, super);
-    rb_vm_add_root_module(klass);
+    rb_vm_register_global_object(klass);
     return klass;
 }
 
@@ -1099,11 +1105,11 @@ rb_define_module(const char *name)
                      name, rb_obj_class(module));
         }
         /* Module may have been defined in Ruby and not pin-rooted */
-        rb_vm_add_root_module(module);
+        rb_vm_register_global_object(module);
         return module;
     }
     module = rb_module_new();
-    rb_vm_add_root_module(module);
+    rb_vm_register_global_object(module);
     rb_const_set(rb_cObject, id, module);
 
     return module;
@@ -1128,13 +1134,13 @@ rb_define_module_id_under(VALUE outer, ID id)
                      outer, rb_id2str(id), rb_obj_class(module));
         }
         /* Module may have been defined in Ruby and not pin-rooted */
-        rb_gc_register_mark_object(module);
+        rb_vm_register_global_object(module);
         return module;
     }
     module = rb_module_new();
     rb_const_set(outer, id, module);
     rb_set_class_path_string(module, outer, rb_id2str(id));
-    rb_gc_register_mark_object(module);
+    rb_vm_register_global_object(module);
 
     return module;
 }