Language feature: file I/O with `open()`

The current interpreter can't interact with the file system, but implementing `open` will solve that.

Support for reading, writing and appending to files will be needed.

```python
file = open('foo.txt')
contents = file.read()
print(contents)
file.close()

file = open('bar.txt', 'w')
chars = file.write(contents)
print("Wrote", chars, "chars")
file.close()
```