`__bool__` should fall back to `__len__` when it is undefined

Hello! I'm trying to use Codon.

According to , if a class does not define `__bool__`, Python should call `__len__` to determine truthiness.

However, in Codon 0.19.3, attempting to evaluate such an object in a boolean context causes an error.

Example:

```python
class A:
def __len__(self) -> int:
return 0

a = A()
print(bool(a))
```

This should output:

```
False
```

But Codon raises an error:

```
bool.codon:12 (16-31): error: 'A' object has no attribute '__bool__'
╰─ Main.py:6 (7-14): error: during the realization of __new__(what: A)
```