Boa: magic functions are not working as expected

[rickycao-qy]

This bug is likely related to #61
Now calling magic function directly is working file. But use system built-in function to call it is giving errors. For example

const boa = require('../packages/boa');
const torch = boa.import('torch');
const {len} = boa.builtins();

class customDataset extends torch.utils.data.Dataset {
  __len__() {
    return 5;
  }
  __getitem__(index) {
    return 2;
  }
}

const dataset =  new customDataset();
console.log(dataset.__len__());  # This is wokring fine
console.log(len(dataset));  # This gives errors

Accordingly in raw python, this is working:

import sys
import torch

class customDataset( torch.utils.data.Dataset ):
  def __len__(self): 
    return 5
  
  def __getitem__(self, index): 
    return 2
  

dataset = customDataset()

print(dataset.__len__())
print(len(dataset))

[yorkie]

It seems that some operations in class initialization are lost.