def subscript_assign(subscript, value)
      clear_memo(:[], subscript)
      if value.is_a?(Base)
        instance[subscript] = value.instance
      else
        instance[subscript] = value
      end
    end
def subscript_assign(subscript, value)
      clear_memo(:[], subscript)
      if value.is_a?(Base)
        instance[subscript] = value.instance
      else
        instance[subscript] = value
      end
    end
assigns a subscript, taking care of memoization and unwrapping a JSI if given.
 @param subscript [Object] the bit between the [ and ]
 @param value [JSI::Base, Object] the value to be assigned