allow to access ivars of classes/modules

[ko1]

if an ivar of a class/module refer to a shareable object, this ivar
can be read from non-main Ractors.

[ko1]

prelude: |
  class C
    @a = 1
    def self.a
      _a = @a; _a = @a; _a = @a; _a = @a; _a = @a;
      _a = @a; _a = @a; _a = @a; _a = @a; _a = @a;
    end
  end
benchmark:
  vm_ivar_of_class: |
    a = C.a
loop_count: 30000000

                  vm_ivar_of_class
               master:   6665015.5 i/s
             modified:   5799075.6 i/s - 1.15x  slower

This patch makes ivar of class x1.15 slower. I think it is acceptable.
No other impact (set/get on the following are not stable).

Calculating -------------------------------------
                        modified      master
         vm_ivar_init     7.159M      7.223M i/s -     30.000M times in 4.190535s 4.153643s
     vm_ivar_of_class     5.824M      6.671M i/s -     30.000M times in 5.151327s 4.496804s
vm_ivar_init_subclass     2.272M      2.297M i/s -      3.000M times in 1.320403s 1.306288s
          vm_ivar_set   154.100M    168.416M i/s -     30.000M times in 0.194678s 0.178131s
              vm_ivar   138.971M    137.498M i/s -     30.000M times in 0.215872s 0.218185s

Comparison:
                      vm_ivar_init
               master:   7222575.1 i/s
             modified:   7158991.2 i/s - 1.01x  slower

                  vm_ivar_of_class
               master:   6671404.9 i/s
             modified:   5823742.4 i/s - 1.15x  slower

             vm_ivar_init_subclass
               master:   2296584.0 i/s
             modified:   2272032.8 i/s - 1.01x  slower

                       vm_ivar_set
               master: 168415514.3 i/s
             modified: 154100401.3 i/s - 1.09x  slower

                           vm_ivar
             modified: 138971409.5 i/s
               master: 137497896.1 i/s - 1.01x  slower

[ko1]

             vm_ivar_of_class_set
              master:   7296353.0 i/s
            modified:   6880438.4 i/s - 1.06x  slower

setter is x1.06 slower.

[eregon]

Suggested change

	Otherwize, only the main Ractor can access instance variables of shareable objects.
	Otherwise, only the main Ractor can access instance variables of shareable objects.

[eregon]

Looks good to me.